A comprehensive MCAT guide to important functional groups, featuring explanations, example questions, and practice problems with step-by-step solutions.
(Note: This guide is part of our MCAT Organic Chemistry series.)
Table of Contents
Part 1: Introduction to functional groups
Part 2: Oxygen-containing groups
a) Alcohols
b) Aldehydes and ketones
c) Carboxylic acids
d) Carboxylic acid derivatives
Part 3: Nitrogen-Containing Groups
a) Amides and amines
b) Imines, and enamines
c) Cyanohydrins
Part 4: Hydrocarbon Functional Groups
a) Hydrocarbon functional groups
b) Amino acid side chains
c) Aromatic compounds
Part 5: Passage-Based Questions and Answers
Part 6: Standalone Questions and Answers
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Part 1: Introduction to functional groups
To gain an understanding of organic chemistry, one must first gain an understanding of functional groups. After all, organic chemistry is all about the study of how molecules interact, and functional groups are collections of atoms that confer certain chemical properties to the molecule.
The sheer variety of functional groups can be overwhelming, but don't worry if you aren't familiar with them yet! This guide will review the major functional groups covered on the MCAT. As you work through the guide, it will be most helpful to determine if these functional groups are electrophilic or nucleophilic. (For a review of the function of electrophiles and nucleophiles, be sure to refer to our guide on the fundamentals of organic chemistry. While the MCAT will rarely test you on nomenclature and molecular structure, general trends in electrophilicity, nucleophilicity, and electronegativity will be the most important information to retain for test day.
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Part 2: Oxygen-containing groups
a) Alcohols
Alcohols are molecules that contain a hydroxyl group, or -OH.
There are quite a few important properties that alcoholic functional groups have. One of the most interesting properties is their acidity. Alcohols can serve as weak Bronsted acids, with a pKa between 15 and 20. Though they have a high pKa, alcohols are able to donate their protons in basic solutions. When this occurs, the hydroxyl group can now act as a nucleophile.
Alcohols can also participate in hydrogen bonding. Recall that hydrogen bonding relies on the interactions donated by hydrogens bound to other elements of high electronegativity (including oxygen, nitrogen, and fluorine). Because alcohols possess a hydroxyl group with a hydrogen atom bound to an oxygen atom, they can participate in hydrogen bonding. This is an important property to consider because hydrogen bonding can raise the boiling points of many alcohols.
Alcohols also participate in many important synthesis reactions. Alcohols can be converted into higher-level functional groups through oxidation reactions. There are specific reagents that can create aldehydes, ketones, and even carboxylic acids from an alcohol.
Figure 1 Examples of reactions involving alcohols.
Along with oxidation reactions, alcohols can participate in substitution reactions (Sn1 and Sn2 reactions). In these reactions, the hydroxyl group can serve as a nucleophile, attacking a carbon center and creating a new molecule. Since hydroxyl groups are able to lose a hydrogen ion in basic conditions, these substitution reactions proceed well when occurring in a basic solution.
Alcohols can also partake in protection reactions and mesylate/tosylate reactions. Chemists perform these reactions to ensure the hydroxyl group does not react or interfere during the desired synthesis reaction.
This can be accomplished in several ways. Generally, chemists turn hydroxyl groups into ketones or aldehydes that become ketals/hemiketals or acetals/hemiacetals. Additionally, hydroxyl groups can be converted into functional groups that are better leaving groups. Mesylate and tosylate reactions convert hydroxyl groups into mesylates or tosylates, which are good leaving groups. These reactions help facilitate substitution or elimination reactions in which an alcohol must be eliminated.
There is a specific nomenclature used to describe alcohols. To do so, determine the name of the alkane body (more on this later). Then, remove the -e ending. A suffix of -ol is added at the end of the name. A number describing which carbon the hydroxyl group is bound to in the longest chain is added as a prefix. Naming alcohols is as simple as that!
Figure 2 This alcohol is named 2-pentanol.
b) Aldehydes and ketones
Both aldehydes and ketones are important functional groups due to their electrophilic nature. The oxygen atom pulls electron density away from the carbon atom, creating a partial positive charge on the carbon atom. This creates a vulnerable site that can be attacked by a nucleophile.
Thus, aldehydes and ketones are commonly involved in nucleophilic addition reactions. These reactions yield many additional functional groups that behave similarly.
Ketal/hemiketal and acetal/hemiacetal reactions are used to protect ketones and aldehydes from participating in undesired reactions. These reactions change the structure of the ketone and aldehyde such that they no longer possess a vulnerable, electrophilic carbon atom.
Ketals are the product of ketones going through two moles of alcohol attack, while a hemiketal has only undergone one mole of alcohol attack. There’s a similar relationship between acetals and hemiacetals, the key distinction being that they are formed from aldehydes. These reactions are also reversible, making this method a great way of forming protecting groups.
Figure 3 Reactions that create ketals/hemiketals and acetals/hemiacetals.
Aldehydes are susceptible to oxidation reactions. Recall that alcohols can be transformed into aldehydes or ketones. Similarly, aldehydes can be converted into carboxylic acids using the same reactions. The oxidation of aldehydes results in functional groups of a higher oxidation state, such as carboxylic acids. (For more information on this, be sure to refer to our guide on oxidation and reduction reactions.)
There are times where adding a base or acid to a ketone can change the structure of the ketone. The reason is that alpha hydrogens are unusually acidic. Alpha hydrogens are hydrogens attached to the carbons adjacent to the electrophilic carbons in a carbonyl. The addition of a base creates a species known as an enolate whereas the addition of small quantities of acid creates a species known as an enol.
When an enol forms, it may spontaneously become a ketone again, which is known as keto-enol tautomerism. The difference between these two species is that a ketone is electrophilic, while an enol is nucleophilic (due to the location of the double bond). Thus, enols tend to attack electrophilic sites.
Figure 4 An example of keto-enol tautomerization
Lastly, an aldol reaction is a way of creating a carbon-carbon bond between two different molecules. A retro-aldol reaction is a method of breaking a carbon-carbon bond in a manner opposite of an aldol reaction. We’ve included the mechanism for the reactions below. Study them carefully to understand how enols and enolates are involved.
Figure 5 Additional reactions involving aldehydes and ketones.
It is important to note that the reactivity of ketones and aldehydes is highly dependent on the substituents located near these functional groups. For instance, if there are elements of low electronegativity that can donate electron density, then the C=O bond may not be as reactive as usual. Similarly, if there are electron-withdrawing groups (typically functional groups with high electronegativity) present, the bond may become even more reactive.
Steric hindrance is another important factor that affects reactivity. In comparison to ketones, it is often easier for reactants to access aldehydes. This is because aldehydes are located at the ends of molecules. Bulky molecules may have lower reactivities with ketones since there are likely carbon atoms and other substituents creating interference.
To name aldehydes, determine the name of the parent alkane and remove the -e suffix. In its place, add the -al suffix. Add a numeric prefix to the name if the aldehyde isn’t the highest order functional group (otherwise, the aldehyde will always be the first carbon in the chain).
The nomenclature for ketones is similar. However, instead of adding an -al suffix, the -one suffix is used.
c) Carboxylic acids
Carboxylic acids are functional groups characterized by the presence of a -COOH group. This functional group is equivalent to a carbonyl with a hydroxyl group attached to the carbon. Carboxylic acids have some important properties related to those already discussed.
Figure 6 Methanoic acid (also known as formic acid), the simplest of carboxylic acids
Carboxylic acids have an electrophilic carbon that is susceptible to nucleophilic attack, as well as a hydroxyl group capable of hydrogen bonding. In addition, there are alpha-hydrogens on the carbon adjacent to the carbonyl carbon. (Remember, these alpha-hydrogens are unusually acidic.) Thus, carboxylic acids can participate in a variety of reactions.
Let’s focus on the carbon center, which is susceptible to nucleophilic attack. As a result, different substrates can attack the carbon and create carboxylic acid derivatives.
In addition to creating new derivatives through receiving nucleophilic attack, the carbon atom can also serve as a site for carboxylic acid reduction. Carboxylic acids can be reduced to aldehydes and ketones, and eventually alcohols. Similar to reduction reactions, carboxylic acids can also be decomposed into a ketone and carbon dioxide in a reaction known as decarboxylation. For this reaction to occur, a ketone must be present at the beta carbon. If a catalyst is provided, an intramolecular mechanism can break down carboxylic acids.
Figure 7 Decomposition of a carboxylic acid into a ketone and carbon dioxide.
Substitution reactions can also occur at the alpha position of carboxylic acids. Recall that carboxylic acids have unusually acidic alpha hydrogens. In fact, these alpha hydrogens can be lost, creating a carbocation at this position. This unstable carbocation is a prime target for nucleophilic attack, effectively resulting in the substitution of the hydrogen by a new functional group.
To name a carboxylic acid, remove the -e suffix from the name of the primary alkane and add the -anoic acid suffix.
d) Carboxylic acid derivatives
Carboxylic acid derivatives include anhydrides, amides, and esters.
A carboxylic acid can serve as a nucleophile, attacking the carbonyl of another carboxylic acid. This process creates what’s known as an anhydride. Thus, an anhydride can be considered as two carboxylic acids bonded together by a shared hydroxyl group.
Figure 8 The structure of acetic anhydride, one of the simplest anhydride compounds that can be formed.
Anhydrides have a much higher boiling point than carboxylic acids. Why is this? Since they are bigger in size than their corresponding acid, they have a greater surface area and ability to engage in Van der Waals force of attractions. Anhydrides are also the most reactive of the three molecules discussed.
The attack of the carbonyl by an alcohol can create an ester. Esters are produced by replacing the hydroxyl groups with an -OR group through a condensation reaction. Then, an intramolecular attack can create a cyclic compound known as a lactone.
Figure 9 An example of a lactone.
Esters tend to be as reactive as carboxylic acids. Esters also tend to have lower boiling points, as they are generally unable to form hydrogen bonds and thus have fewer intermolecular attractions.
Amides are the least reactive molecule discussed in this section. Amides are a product of an amine attacking the carbonyl. An amine is a nitrogenous functional group with a lone pair of electrons. This lone pair of electrons can replace the hydroxyl groups through a condensation reaction. Occasionally, the resulting product can perform an intramolecular attack that creates a cyclic compound known as a lactam.
Figure 10 An amide undergoing cyclization to form a lactam.
Anhydrides, esters, and amides are all viable reactants for nucleophilic substitution since they each have an electrophilic carbonyl atom. Recall that anhydrides tend to be more reactive than esters, while esters tend to be more reactive than amides. Depending on the substrate, different products can form.
Attack of an anhydride carbonyl can cause the anhydride to split into a carboxylic acid and a carboxylic acid/derivative.
Esters have the potential to also break down into their respective alcohol and a carboxylic acid or be replaced by another alcohol in a process known as transesterification.
Amides can be hydrolyzed in a manner that reverses their formation in the presence of aqueous acid.
Keep in mind the relative reactivities of these functional groups. There are also steric strain and electronic effects that must be considered in these molecules.
Steric strain refers to the ability of molecules to access a particular site on another molecule. Because anhydrides, esters, and amides are larger molecules, accessing their carbonyl carbons can be difficult for large bulky molecules. As a result, smaller nucleophiles may have more success.
Electronic effects help to explain the reactivity trends we see. Because oxygen is a highly electronegative atom, oxygen atoms “pull” more strongly on the electrons of neighboring atoms. As a result, a dipole is formed. Any adjacent carbonyl carbon thus has a partial positive charge that makes it electrophilic.
To name anhydrides, remove the -acid suffix and add an -anhydride suffix for symmetrical anhydrides. For anhydrides that are unsymmetrical, name each of the carboxylic acids that contributed to the anhydride formation. To name amides, remove the -oic acid suffix and add an -amide ending. Finally, to name esters, first name the alkyl chain that was most recently added to the parent chain. Then, remove the -e suffix from the parent chain and add an -oate suffix.
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Part 3: Nitrogen-containing groups
a) Amides and amines
The nitrogen-containing groups include amides, amines, imines, and enamines. These functional groups have been briefly discussed above in the context of derivatives of other functional groups, such as carboxylic acids.
Amides are nitrogen functional groups that consist of hydrogen bonds and a carbon carbonyl bond to the nitrogen. Because of this structure, most amides (primary and secondary amides, but not tertiary) are able to form hydrogen bonds. Because of this ability to hydrogen bond, amides have high boiling and melting points.
To name amides, drop the -ic acid ending of the parent carboxylic acid molecule. Then, add the word amide at the end. The carbonyl carbon the amide is bonded to is the primary carbon in the naming chain.
Amines are usually small and not bound to carbons that are a part of carbonyl bonds. The structure of amines is usually NRx. Primary amines consist of one carbon bond, while secondary amines consist of two carbon bonds. Tertiary amines consist of three carbon bonds and are unable to participate in hydrogen bonding. Because of hydrogen bonding and polarity, amines are soluble in water and have high boiling and melting points similar to amides. Amines are responsible for the nucleophilic attack of electrophilic carbons that create amides.
Figure 11 Amines and amides. R indicates the "rest of the molecule."
b) Imines and enamines
Sometimes, nucleophilic attacks can create species known as imines and enamines. Imines are functional groups in which the oxygen in a carbonyl bond is replaced by a nitrogen atom. As a result, imines also behave similarly to carbonyl functional groups.
Enamines are a resonance form of imines. If an alpha hydrogen is removed from an imine group, then a double bond forms between the carbon adjacent to the carbonyl group and the carbonyl carbon. This double bond forces a rearrangement of electrons, breaking one of the double bonds with nitrogen and forcing it to pick up another molecule to bind to, preferably a hydrogen atom. This interconversion favors the enamine due to its relatively stable structure.
Figure 12 Interconversion between imines and enamines.
c) Cyanohydrins
Cyanohydrins are molecules that contain two distinct functional groups: a cyano functional group (CN) and a hydroxyl functional group (OH). Note that within the cyano functional group, the carbon and nitrogen atoms are triple-bonded together.
Cyanohydrins are typically formed through nucleophilic addition when a cyano group attacks a carbonyl functional group. They are also intermediates in amino acid formation, such as during Strecker synthesis.
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Part 4: Hydrocarbon functional groups
a) Hydrocarbon functional groups
Hydrocarbons are chained molecules formed by carbon and hydrogen. They can be classified by the types of bonds present in their chains. Molecules with only single bonds between hydrocarbons are alkanes and will have the suffix -ane. Molecules with one or more double bonds are alkenes and will have the suffix of -ene. Finally, molecules with one or more triple bonds are alkynes and will have the ending of -yne.
Figure 13 Comparison of 4-carbon alkane, alkene, and alkyne.
The first step in naming hydrocarbons is to identify the types of bonds present in the molecules. Molecules with only single bonds between hydrocarbons are alkanes and will have the suffix -ane. Molecules with one or more double bonds are alkenes and will have the suffix of -ene. Finally, molecules with one or more triple bonds are alkynes and will have the ending of -yne.
b) IUPAC Naming System
Consider the molecule below. You may recognize several of the functional groups present in its structure, but how would you construct a name for it?
Figure 14 A molecule with many carbons and several recognizable functional groups.
Organic molecules are named using a set of rules outlined by the IUPAC, the world authority on chemical nomenclature. Let’s follow these guidelines to construct a name for our molecule. The first step is to figure out the numerical prefix for the name. This is based on the number of carbons in the longest continuous and highest priority carbon chain, or the parent chain of the molecule.
Table 1 IUPAC prefixes for carbon chains
meth- |
eth- |
prop- |
but- |
pent- |
hex- |
hept- |
oct- |
non- |
dec- |
undec- |
dodec- |
To determine priority, you must first identify if the molecule has functional groups. If there are multiple, the one with the highest priority is the ‘parent structure’ and determines the suffix of the molecule’s name. Once you have found the highest priority group, number the carbons of the parent chain such that this group is located at the lowest possible number. If there are no functional groups present, simply find the longest continuous alkane chain and assign carbon numbers such that the longest hydrocarbon substituent group is at the lowest possible number. The table below summarizes the priority rankings and associated suffixes for the most common functional groups on the MCAT - as a general rule, the more oxidized a group is, the higher its priority rank will be.
Table 2 The alkyl halide substituent name depends on the halogen. For example, Bromine's substituent name is "Bromo- '' and Chlorine's is "Chloro-".
Once we know the appropriate prefix and suffix for the parent chain, we can use the following formula to form the basis of our molecule’s name: prefix + “an” + suffix. Therefore, our molecule is an octanoic acid based on its parent chain.
Figure 15 Based on the components of its parent chain, this molecule is an "octanoic acid".
Now we have the core of our molecule’s name. The remainder of its name is based on the other functional groups and carbon side chains in the molecule, which are known as substituent groups. First, name the carbon groups that are not part of the parent chain. Similar to our naming scheme for carbons before, assign them a numerical prefix and attach the suffix -yl, so a -CH3 substituent becomes methyl while a -CH2CH3 substituent becomes ethyl. Substituent functional groups are named based on their associated substituent names.
Figure 16 All substituent groups and names for this octanoic acid.
Once all of the substituent groups have been named, the entire name of the molecule can be assembled. To do so, use the numbers we assigned to carbons in the parent chain to specify where each group is located. Then, list the substituent names in alphabetical order with their carbon number attached using dashes. If there are multiple of the same group in a molecule, separate their carbon numbers with a comma and attach a numerical prefix to their name (ex: -1,7-diene-).
Figure 17 The IUPAC nomenclature for this molecule is 3-ethyl-6-methyl-4-oxo-octanoic acid. Sometimes, the final dash will be left out of molecules' names (ex: "-oxooctanoic acid").
Now that we have named our molecule, let’s briefly review the steps in the IUPAC naming process:
Identify the parent chain of the molecule and form the core of the name
Locate and assign names to all substituent groups
Organize them alphabetically by their substituent names
Add them to the name with dashes and their associated parent chain carbon numbers
You should now be well equipped to systematically construct names for many organic molecules. However, there are still some additional special cases that may show up on the MCAT. Parent chains containing alkenes or alkynes should be named with the following formula: prefix + “ene” or “yne”. If a parent chain contains both of these groups, it will have the suffix format: “#-en-#-yne”. Also, ring molecules have the prefix “cyclo-” added to their parent chain names. Finally, the following designations for groups may be listed at the start of a molecule’s name: E/Z for alkenes, R/S for chiral carbons, and ortho/meta/para for benzene derivatives.
Figure 18 Special cases in chemical nomenclature.
c) Side chains of amino acids
If you’ve studied amino acids, then you are already familiar with many different functional groups! The side chains of amino acids include a variety of functional groups, from simple alkanes and hydroxyl groups to isopropyl, benzene, and thiol groups.
Isopropyl functional groups are hydrocarbon functional groups in which two carbons are bonded to a single carbon. This functional group is present in the amino acid valine.
Figure 19 Valine’s R group is an isopropyl functional group
Benzene functional groups are different from anything we’ve encountered so far. Benzene is a circular carbon molecule consisting of six carbons and alternating double and single bonds. Benzene is known to be aromatic because of this alternating bond structure. Thus, benzene rings tend to be very stable and can engage in pi-stacking interactions.
When bonded to another molecule, benzene becomes known as a phenyl ring. Some amino acids with phenyl rings in their side chain are phenylalanine, tryptophan, and tyrosine.
Figure 20 Phenylalanine, tyrosine and trypotophan have phenyl rings in their R groups
Thiol functional groups consist of a sulfur atom bound to a hydrogen atom. These functional groups are present in the amino acid cysteine. These thiol groups are extremely useful in forming disulfide bonds that provide structure to protein.
Figure 21 Cytosine has a thiol as its R group
Here, we’ve focused on only a small subset of amino acids. For more information on amino acids and their functional groups, be sure to refer to our guide on proteins.
d) Aromatic compounds
Aromatic compounds play an important role in biological functions and reactions. Recall that aromatic compounds are often planar and contain 4n+2 electrons in a conjugated electron system.
Polycyclic and heterocyclic aromatic compounds can be found in nucleic acids. Take a look at the following structures of purines and pyrimidines.
Figure 22 The four nucleotide bases used in DNA: adenine, thymidine, guanosine, and cytidine
The cyclic nature of DNA bases plays an important role in the bonding and stability of the molecule. For more information on this, be sure to refer to our guide on DNA.
While a phenyl functional group refers to the presence of a single aromatic benzene ring, a phenol functional group contains a hydroxyl group attached to a benzene ring. Biomolecules with phenol functional groups, such as hydroquinone and ubiquinone, are common sites of oxidation and reduction reactions.
Figure 23 Hydroquinone and a ubiquinol, both molecules with a phenol functional group.
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Part 5: Passage-based questions and answers
The oxidation of an alcohol via PCC was applied to form 1-(3,4-methylenedioxy phenyl)-2-propanone (reaction 1).
Rection 1
Reaction 2
This passage was created for educational purposes only
Question 1: Which of the following is true about alcohols?
A) They are weak Bronsted bases
B) They can participate in hydrogen bonding
C) Their name includes the suffix -one
D) They are nonpolar
Question 2: The reactants in reaction 1 contain which of the following types of alcohols?
A) Methanol
B) Primary alcohols
C) Secondary alcohols
D) Tertiary alcohols
Question 3: The molecule formed by the Jones oxidation contains which of the following functional groups?
A) Carboxylic acid
B) Aldehyde
C) Ketone
D) Primary alcohol
Question 4: The product formed by reaction 1 contains which of the following functional groups?
A) Aldehyde
B) Alcohol
C) Ketone
D) Amide
Question 5: Suppose the product of reactant 2 were to adopt a straight-chain form. What is the name of the resulting molecule?
A) 1-hexanoic acid
B) Hexanal
C) 1-pentanoic acid
D) Pentanal
Answer key for passage-based practice questions
Answer choice B is correct. Alcohols contain hydroxyl groups that are able to participate in hydrogen bonding. This allows them to interact with water, making them polar (choice D is incorrect). Alcohols are considered to be weak Bronsted acids (choice A is incorrect). Alcohol nomenclature involves the suffix -ol. The suffix -one is used in naming ketones (choice C is incorrect).
Answer choice C is correct. The reactant in figure 1 contains a hydroxyl group bonded to a carbon that is bonded to two other substituents. Thus, it is a secondary alcohol (choice C is correct). Methanol is not a class of alcohols. It is the compound CH₃OH, the simplest alcohol (choice A is incorrect). Primary alcohols contain a carbon bonded to only one other carbon (choice B is incorrect). Tertiary alcohols contain a carbon bonded to three other carbons (choice D is incorrect).
Answer choice A is correct. Referring to reaction 2, note that the Jones oxidation forms a carboxylic acid (R-COOH). Aldehydes and ketones would be formed using a weaker oxidizing agent (choices B and C are incorrect). A primary alcohol serves as the reactant for the Jones oxidation (choice D is incorrect).
Answer choice C is correct. Referring to reaction 1, note that a ketone is formed (R₂C=O) (choice C is correct). An aldehyde would have a hydrogen connected to the carbonyl carbon (choice A is incorrect). An alcohol (-OH) was the reactant that was converted into the ketone (choice B is incorrect). Amides are a nitrogen-containing functional group. They have a carbonyl carbon bonded to a nitrogen atom (choice D is incorrect).
Answer choice A is correct. In its straight-chain form, the product of reaction 2 would be the following molecule:
It is a six-carbon chain with a carboxylic acid functional group (choice A is correct). Hexanal is a molecule with a six-carbon chain but contains an aldehyde functional group (choice B is incorrect).
1-pentanoic acid contains the carboxylic acid functional group, but lacks one carbon atom from the parent chain (choice C is incorrect).
Pentanal does not contain a carboxylic acid group. Instead, it contains an aldehyde group. Additionally, it lacks one carbon atom from the parent chain (choice D is incorrect).
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Part 6: Standalone questions and answers
Question 1: Which type of reaction can create a carboxylic acid out of an alcohol?
A) Substitution
B) Elimination
C) Reduction
D) Oxidation
Question 2: Which answer choice correctly orders the reactivity of the following from least reactive to most reactive: alcohols, esters, anhydrides, and amides?
A) Amides, esters, anhydrides, and alcohol
B) Alcohol, esters, amides, and anhydrides
C) Alcohol, amides, esters, and anhydrides
D) Anhydride, esters, amides, and alcohol
Question 3: Which of the following amino acids contains a phenyl ring within its side chain?
A) Phenylalanine
B) Alanine
C) Serine
D) Histidine
Question 4: Which of the following statements most accurately characterizes the carbon atom in a carbonyl group?
A) The oxygen atom donates electron density
B) The oxygen atom withdraws electron density
C) The carbon is a carbocation
D) The carbon is a carbanion
Question 5: Why is steric strain an important concern when considering the products of organic reactions?
A) Steric strain might prevent access to an electrophilic site
B) Steric strain might create an imbalance of electron density
C) Steric strain determines the stability of carbocation intermediates
D) Steric strain determines the reactivity of a compound
Answer key for standalone questions
1. Answer choice D is correct. Alcohols are able to become more complex functional groups by oxidizing (choice D is correct). The reverse process is reduction (choice C is incorrect). An easy way to remember this is that oxidation occurs as the number of bonds to oxygen increases, while reduction involves reducing those bonds. Substitution and elimination are broad types of reactions that are not specific enough for the scope of this question (choices A and B are incorrect).
2. Answer choice C is correct. This question requires knowledge of the reactivity of all of the functional groups. Anhydrides are the most reactive of any functional groups (choices A and D are incorrect). Esters are more reactive than amides due to the presence of a highly electronegative oxygen atom (choice C is correct).
3. Answer choice A is correct. This question tests how well you know the structure of amino acids. For the MCAT, you should have the structures memorized. Only phenylalanine has a benzene functional group in its side chain (also known as a phenyl ring). The other listed amino acids have either a hydroxyl, hydrocarbon, or another ringlike functional group.
4. Answer choice B is correct. Understanding why a carbonyl carbon is so reactive is crucial to understanding functional group reactivities. A carbonyl group is a carbon double bonded to an oxygen atom. Oxygen is a strong electron attractor. Thus, oxygen withdraws electron density away from the carbon atom, creating a partial positive charge (choice B is correct).
5. Answer choice D is correct. Steric strain refers to the tension between bonds in a molecule. Because steric strain is a physical constraint instead of an electronic concern, answer choice B can be eliminated. Although steric strain may block access to certain sites on a molecule, this is more accurately described by hindrance (choice A is incorrect). Steric strain pertains solely to tension within bonds. In molecules with high steric strain, the molecule is more reactive than normal, since the molecule exists with a higher potential energy. As a result, a smaller amount of additional energy is required to reach a transition state and participate in a chemical reaction (choice D is correct).
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