Functional Groups and IUPAC

Question 1

What are the 3 primary rules of naming an organic compound according to the IUPAC nomenclature scheme?

Answer 1

1. Identify the molecule’s principal functional group and find the longest carbon chain containing that functional group.
2. Number the carbon chain from one end such that the principal functional group receives the lowest possible number.
3. Name all other functional groups according to this numbering scheme.

Question 2

What is the prefix for carbon chains consisting of:

• 1 carbon?
• 2 carbons?

Answer 2

• 1 carbon = “meth-“
• 2 carbons = “eth-“

Question 3

What is the prefix for carbon chains consisting of:

• 3 carbons?
• 4 carbons?

Answer 3

• 3 carbons = “prop-“
• 4 carbons = “but-“

Question 4

What is the prefix for carbon chains consisting of:

• 5 carbons?
• 6 carbons?

Answer 4

• 5 carbons = “pent-“
• 6 carbons = “hex-“

Question 5

What is the prefix for carbon chains consisting of:

• 7 carbon?
• 8 carbons?

Answer 5

• 7 carbon = “hept-“
• 8 carbons = “oct-“

Question 6

What is an alkane?

Answer 6

An alkane is an organic molecule consisting of nothing but carbon and hydrogen atoms, connected with only single bonds.

All alkanes are named with an -ane suffix, with the prefix determined by the number of carbons in the chain. The three carbon chain below is called propane.

Question 7

What is the name of this molecule?

Answer 7

This molecule is named pentane.

To name an alkane, identify the prefix based on the number of carbons in the chain containing the largest functional group, then attach the -ane suffix.

Question 8

What is the name of this molecule?

Answer 8

This molecule is iso-butane, or 2-methyl-propane.

Historically, alkane chains which are branched at the very end have been named with the iso- prefix. While this may show up in passages, it is very rarely tested on the MCAT, since IUPAC has moved away from that old naming convention into the new system.

Question 9

What is an alkene?

Answer 9

An alkene is an organic molecule consisting of nothing but carbon and hydrogen atoms, containing at least one carbon-carbon double bond.

All alkenes are named with an -ene suffix, with the prefix determined by the number of carbons in the chain. The molecule below is propene.

Question 10

What is the name of this molecule?

Answer 10

This molecule is named 2-butene.

To name an alkene, identify the prefix based on the number of carbons in the chain containing the largest functional group, then attaching the -ene suffix, along with the number of the lowest-numbered carbon participating in the double bond.

Question 11

What is an alkyne?

Answer 11

An alkyne is an organic molecule consisting of nothing but carbon and hydrogen atoms, containing at least one carbon-carbon triple bond.

All alkynes are named with an -yne suffix, with the prefix determined by the number of carbons in the chain. The molecule below is propyne.

Question 12

What is the name of this molecule?

Answer 12

This molecule is named 1-butyne.

To name an alkyne, identify the prefix based on the number of carbons in the chain containing the largest functional group, then attaching the -yne suffix, along with the number of the lowest-numbered carbon participating in the triple bond.

Question 13

What is the principal functional group in this molecule?

Answer 13

The amine -NH₂ is the principal functional group.

In this molecule, 1-propanamine, -NH₂ is the only functional group attached to the main carbon chain.

Question 14

What is the principal functional group in this molecule?

Answer 14

The -OH group is the principal functional group.

In general, oxygen-containing groups take priority in nomenclature, and the more bonds to oxygen in the functional group, the higher its priority. This molecule, 1-amino-2-propanol, has the -ol ending to indicate highest priority even though the amino group is on the 1 carbon.

Question 15

At what position does the highest-priority functional group reside in this molecule?

Answer 15

The -OH is on the 1 carbon position.

By definition, the chain is numbered to give the highest priority functional group the lowest possible number. Hence any time a group is at the end of the chain, it is in the 1 position by default. This would always be called 1-butanol, never 4-butanol.

Question 16

At what position does the highest-priority functional group reside in this molecule?

Answer 16

The highest priority group is on the 2 carbon position.

Remember: oxygen-containing functional groups take priority over nitrogen-containing groups. The chain is numbered so that the -OH group resides at the lowest possible numerical position, making this a 4-amino-2-butanol molecule.

Question 17

At what position does the Cl group reside in this molecule?

Answer 17

The Cl is on the 3 carbon position in this molecule.

By definition, the chain is numbered so that the highest-priority functional group gets the lowest possible number. In this molecule, 3-chloro-1-butanol, the -OH must be position 1 by default.

Question 18

At what position does the Cl group reside in this molecule?

Answer 18

The Cl group is on the 3 carbon position.

When it comes to cyclic carbon chains, still number so that the highest priority group gets the lowest possible number, by default this is 1. Then, label in whichever direction results in the lowest possible number for the remaining functional groups. This molecule is 3-chlorocyclohexanol and would never be called 5-chlorocyclohexanol.

Question 19

What is an alcohol?

Answer 19

An alcohol is an organic molecule which has an -OH as the highest-priority functional group.

All alcohols are named with an -ol suffix, with the prefix determined by the number of carbons in the chain. The molecule below is propanol.

Question 20

What is the name of this molecule?

Answer 20

This molecule is named 2-butanol.

To name an alcohol, identify the prefix based on the number of carbons in the chain containing the -OH functional group, then attaching the -ol suffix, along with the number that makes the -OH on as low a carbon as possible. This would never be called 3-butanol.

Question 21

What is the name of this molecule?

Answer 21

This molecule is named phenol, or benzenol.

While the second name follows the IUPAC system more closely, phenol is the name that the MCAT will use. This follows from there being a phenyl group (-C₆H₅) attached to the -OH group.

Question 22

What is an aldehyde?

Answer 22

An aldehyde is an organic molecule consisting of an alkane chain with a carbon-oxygen double bond on one terminal carbon of the chain.

All aldehydes are named by removing the -e from the end of the name of the parent alkane and replacing it with -al. The three carbon chain below is called propanal.

Question 23

What is the name of this molecule?

Answer 23

This molecule is named pentanal.

To name an aldehyde, remove the -e from the end of the name of the parent alkane and replace it with -al. Since an aldehyde is a terminal group (always position 1) it’s unnecessary to write 1-pentanal.

Question 24

What is a ketone?

Answer 24

An ketone is an organic molecule consisting of an alkane chain with a carbon-oxygen double bond on one of the internal carbons of the chain.

All ketones are named by removing the -e from the end of the name of the parent alkane and replacing it with -one. The three carbon chain below is called propanone, also known commonly as acetone.

Question 25

What is the name of this molecule?

Answer 25

This molecule is named 2-pentanone.

To name an ketone, remove the -e from the end of the name of the parent alkane and replace it with -one. This will always be called 2-pentanone and never 4-pentanone.

Question 26

What is a carboxylic acid?

Answer 26

An carboxylic acid is an organic molecule consisting of an alkane chain with a carbon-oxygen double bond and a hydroxyl (-OH) on one of the terminal carbons of the chain.

All carboxylic acids are named by removing the -e from the end of the name of the parent alkane and replacing it with -oic acid. The three carbon chain below is called propanoic acid.

Question 27

What is the name of this molecule?

Answer 27

This molecule is named butanoic acid.

To name a carboxylic acid, remove the -e from the end of the name of the parent alkane and replace it with -oic acid.

Question 28

What is a dicarboxylic acid?

Answer 28

A dicarboxylic acid is a molecule with two carboxylic acid functional groups.

If the molecule is a chain (rather than a ring), the acids will be at both ends of the chain. The molecule below is propanedioic acid.

Question 29

What is an acid chloride?

Answer 29

An acid chloride is a carboxylic acid derivative, where the -OH has been replaced by a -Cl.

All acid chlorides are named by removing the -oic acid from the end of the name of the parent carboxylic acid and replacing it with -oyl chloride. The three carbon chain below is called propanoyl chloride.

Question 30

What is the name of this molecule?

Answer 30

This molecule is named pentanoyl chloride.

To name an acid chloride, remove the -oic acid from the end of the name of the parent carboxylic acid and replace it with -oyl chloride.

Question 31

What is an acid anhydride?

Answer 31

An acid anhydride is the result of dehydration (loss of H₂O) of two carboxylic acid molecules while joining together.

Acid anhydrides are named by adding ‘-oyl’ to the name of the first acid group, and ‘-oate’ to the other. The molecule below is called propanoyl propanoate, also known commonly as propanoic anhydride since both acids are the same.

Question 32

What is the name of this molecule?

Answer 32

This molecule is named pentanoyl pentanoate, also commonly as pentanoic anhydride.

To name an acid anhydride, add ‘-oyl’ to the name of the first acid group, and ‘-oate’ to the other. When both groups are from the same base acid, it’s common to see the name as the carboxylic acid then attach ‘anhydride’.

Question 33

What is an amide?

Answer 33

An amide is a carboxylic acid derivative, where the -OH has been replaced by an -NH₂.

All amides are named by removing the -oic acid from the end of the name of the parent carboxylic acid and replacing it with -amide. The three carbon chain below is called propanamide.

Question 34

What is the name of this molecule?

Answer 34

This molecule is named hexanamide.

To name an amide, remove the -oic acid from the end of the name of the parent carboxylic acid and replace it with -amide.

Question 35

What is an ester?

Answer 35

An ester is a carboxylic acid derivative, where the -OH has been replaced by an -OR, where R is a hydrocarbon chain.

Esters are named by adding the ‘-yl’ suffix to the alkane chain which is not on the carbonyl side of the single-bonded oxygen, and the ‘-oate’ suffix to the original carboxylic acid (carbonyl side of the single-bonded oxygen). The molecule below is propyl ethanoate.

Question 36

What is the name of this molecule?

Answer 36

This molecule is named ethyl pentanoate.

Esters are named by adding the -yl suffix to the alkane chain which is not on the carbonyl side of the single-bonded oxygen, and the -oate suffix to original carboxylic acid (carbonyl side of the single-bonded oxygen).

Question 37

What is a β-keto carboxylic acid?

Answer 37

A β-keto carboxylic acid is a carboxylic acid molecule with a ketone (carbonyl) exactly two carbon atoms away from the carboxylic acid carbon.

The molecule below is β-keto butanoic acid.

Question 38

What reaction are β-keto carboxylic acids best known for?

Answer 38

β-keto carboxylic acids commonly undergo decarboxyation reactions, where the molecule loses a CO₂, and is left as an enol, which rapidly tautomerizes to a ketone.

(image showing decarboxylation followed by tautomerization here)

Question 39

What is an amine?

Answer 39

An amine is an organic chemistry molecule, consisting of an alkane chain where one of the H atoms has been replaced with an NH₂ functional group.

Amines are named by removing the -e suffix from the parent alkane molecule, and adding -amine. The 3-carbon molecule below is propanamine.

Question 40

What is the name of this molecule?

Answer 40

This molecule is named propanamine.

Amines are named by removing the -e suffix from the parent alkane molecule, and adding -amine.

Question 41

What is the name of this functional group?

(I’m thinking a phenyl group bound to a generic R here, for simplicity.)

Answer 41

This is the phenyl functional group.

A phenyl functional group is a benzene ring (C₆H₅) bound directly to the carbon chain.

Question 42

What is the name of this functional group?

(I’m thinking a benzyl group bound to a generic R here, for simplicity.)

Answer 42

This is the benzyl functional group.

A benzyl functional group is a benzene ring (C6H5) bound to the carbon chain through an intermediate CH₂.

Question 43

What is the name of this functional group?

(I’m thinking a nitro group bound to a generic R here, for simplicity.)

Answer 43

This is the **nitro **functional group.

Question 44

What is a thiol?

Answer 44

A thiol is an organic molecule, chemically similar to an alcohol, with the -OH functional group replaced by an -SH.

Since sulfur is in the same group as oxygen, they have similar chemical properties. In general, in organic chemistry. the prefix thio- indicates sulfur-containing.

Question 45

What is an ether?

Answer 45

An ether is an organic molecule with two alkane chains, both single-bonded to a single oxygen atom.

Ethers are named by according to the alkane chains bonded to the oxygen. The molecule below is diethyl ether.

Question 46

What is an acetal?

Answer 46

An acetal is an aldehyde derivative, where the carbonyl has been replaced by two OR groups, where the R groups can be identical or different.

Acetals are formed via the reaction of an aldehyde molecule with two equivalents of alcohol.

Question 47

What is a ketal?

Answer 47

An ketal is a ketone derivative, where the carbonyl has been replaced by two OR groups, where the R groups can be identical or different.

Ketals are formed via the reaction of a ketone molecule with two equivalents of alcohol.

Question 48

What is the name of this functional group?

Answer 48

This functional group is an imine functional group.

Imine nomenclature is rarely tested on the MCAT; it is sufficient to know the identity of the functional group.

Question 49

What is the name of this functional group?

Answer 49

This functional group is a nitrile functional group.

Nitrile nomenclature is rarely tested on the MCAT; it is sufficient to know the identity of the functional group.