Classes of Organic Compounds: Functional Groups and Nomenclature

Classes of Organic Compounds

Overview of Classification

Organic compounds are classified primarily by their functional group, which determines their chemical properties and reactivity. The three broad classes are:

• Hydrocarbons: Compounds composed only of carbon and hydrogen.

• Compounds containing oxygen

• Compounds containing nitrogen

Hydrocarbons

Types of Hydrocarbons

Hydrocarbons are the simplest organic compounds and are divided based on the types of bonds and ring structures present.

• Alkanes: Only single bonds between carbons; all carbons are sp3 hybridized. Example: ethane ($CH_3CH_3$).

• Cycloalkanes: sp3 carbons form a ring. Example: cyclopentane ($C_5H_{10}$).

• Alkenes: Contain at least one carbon-carbon double bond. Example: ethene ($CH_2=CH_2$).

• Cycloalkenes: Double bond present in a ring structure. Example: cyclohexene.

• Alkynes: Contain at least one carbon-carbon triple bond; carbons involved are sp hybridized. Example: ethyne ($HC ext{≡} CH$).

• Aromatic: Follows Hückel’s Rule (see Chapter 16); benzene is the most common example. Aromatic compounds are also called arenes.

Alkane Naming

Systematic Nomenclature

Alkanes are named according to the number of carbon atoms in the longest continuous chain. The following table summarizes the correspondence between prefixes and the number of carbons:

Alkane Name	Number of Carbons
methane	1
ethane	2
propane	3
butane	4
pentane	5
hexane	6
heptane	7
octane	8
nonane	9
decane	10

Branched alkanes and isomers are named using prefixes and locants to indicate substituent positions.

Cycloalkanes

Structure and Nomenclature

Cycloalkanes are a special class of alkanes in which the carbon atoms form a ring. All carbons are sp3 hybridized.

• Cyclopentane: Five-membered ring ($C_5H_{10}$)

• Cyclohexane: Six-membered ring ($C_6H_{12}$)

Alkenes

Structure and Naming

Alkenes are hydrocarbons that contain at least one carbon-carbon double bond. Their names end in the -ene suffix.

• Number the chain so the double bond gets the lowest possible number.

• Indicate the position of the double bond by the number of the first carbon involved.

Examples:

• Ethene (ethylene): $CH_2=CH_2$

• Propene (propylene): $CH_2=CH-CH_3$

• But-1-ene: $CH_2=CH-CH_2-CH_3$

• But-2-ene: $CH_3-CH=CH-CH_3$

Alkynes

Structure and Naming

Alkynes are hydrocarbons that contain at least one carbon-carbon triple bond. Their names end in the -yne suffix.

• Number the chain so the triple bond gets the lowest possible number.

• Indicate the position of the triple bond by the number of the first carbon involved.

Examples:

• Ethyne (acetylene): $HC ext{≡} CH$

• Propyne (methylacetylene): $CH_3C ext{≡} CH$

• But-1-yne: $CH_2=CH-CH_2-CH_3$

• But-2-yne: $CH_3-C ext{≡} C-CH_3$

Aromatic Hydrocarbons

Structure and Examples

Aromatic hydrocarbons, also called arenes, contain conjugated pi electron systems and follow Hückel’s Rule ($4n+2$ pi electrons, where $n$ is an integer). Benzene is the prototypical aromatic compound.

• Benzene: $C_6H_6$

• Ethylbenzene: Benzene ring with an ethyl group ($C_6H_5CH_2CH_3$)

• Substituted benzenes: Alkyl or other groups attached to the benzene ring

Compounds Containing Oxygen

Functional Groups

• Alcohols: Contain the hydroxyl group ($-OH$) as the main functional group.

• Ethers: Contain two alkyl groups bonded to an oxygen atom ($R-O-R'$).

• Aldehydes and Ketones: Contain the carbonyl group ($C=O$). Aldehydes have at least one hydrogen attached to the carbonyl carbon; ketones have two alkyl groups attached.

• Carboxylic acids: Contain the carboxyl group ($-COOH$), which is a combination of a carbonyl and a hydroxyl group.

Alcohols

Structure and Examples

Alcohols are organic compounds that contain the hydroxyl group ($-OH$).

• Methanol (methyl alcohol): $CH_3OH$

• Ethanol (ethyl alcohol): $CH_3CH_2OH$

• Propan-1-ol: $CH_3CH_2CH_2OH$

• Isopropanol (propan-2-ol): $CH_3CH(OH)CH_3$

Ethers

Structure and Examples

Ethers are composed of two alkyl groups bonded to an oxygen atom.

• Diethyl ether: $CH_3CH_2OCH_2CH_3$

• Furan: A five-membered aromatic ring containing one oxygen atom

• Methyl tert-butyl ether: $CH_3OC(CH_3)_3$

• Dimethyl ether: $CH_3OCH_3$

Example: Ethers are commonly used as solvents due to their relatively low reactivity.

Aldehydes and Ketones

Structure and Examples

• Aldehydes: The carbonyl group is bonded to at least one hydrogen. Example: formaldehyde ($HCHO$), acetaldehyde ($CH_3CHO$).

• Ketones: The carbonyl group is bonded to two alkyl groups. Example: acetone ($CH_3COCH_3$), methyl ethyl ketone ($CH_3COC_2H_5$).

Carboxylic Acids

Structure and Examples

Carboxylic acids contain the carboxyl group ($-COOH$) as their functional group.

• Formic acid: $HCOOH$

• Acetic acid: $CH_3COOH$

• Propionic acid: $CH_3CH_2COOH$

• Butyric acid: $CH_3CH_2CH_2COOH$

Carboxylic Acid Derivatives

Types and Structure

Carboxylic acids can be converted to a variety of derivatives, all containing the carbonyl group bonded to an oxygen or another electron-withdrawing element.

• Acid chlorides: $RCOCl$

• Esters: $RCOOR'$

• Amides: $RCONH_2$

Compounds Containing Nitrogen

Functional Groups

• Amines: Alkylated derivatives of ammonia ($NH_3$), with one or more alkyl groups replacing hydrogens. Example: $RNH_2$, $R_2NH$, $R_3N$.

• Amides: Carboxylic acid derivatives with a nitrogen attached to the carbonyl group. Example: $RCONH_2$.

• Nitriles: Contain the cyano group ($-C ext{≡} N$). Example: acetonitrile ($CH_3CN$).

Summary Table: Major Classes of Organic Compounds

Class	Functional Group	General Formula	Example
Alkane	None	$C_nH_{2n+2}$	Ethane ($CH_3CH_3$)
Alkene	Double bond	$C_nH_{2n}$	Ethene ($CH_2=CH_2$)
Alkyne	Triple bond	$C_nH_{2n-2}$	Ethyne ($HC ext{≡} CH$)
Aromatic	Benzene ring	$C_6H_6$	Benzene
Alcohol	Hydroxyl ($-OH$)	$R-OH$	Ethanol ($CH_3CH_2OH$)
Ether	Ether ($R-O-R'$)	$R-O-R'$	Diethyl ether ($CH_3CH_2OCH_2CH_3$)
Aldehyde	Carbonyl ($-CHO$)	$R-CHO$	Acetaldehyde ($CH_3CHO$)
Ketone	Carbonyl ($>C=O$)	$R-CO-R'$	Acetone ($CH_3COCH_3$)
Carboxylic acid	Carboxyl ($-COOH$)	$R-COOH$	Acetic acid ($CH_3COOH$)
Amine	Amino ($-NH_2$)	$R-NH_2$	Methylamine ($CH_3NH_2$)
Amide	Amide ($-CONH_2$)	$R-CONH_2$	Acetamide ($CH_3CONH_2$)
Nitrile	Cyano ($-C ext{≡} N$)	$R-CN$	Acetonitrile ($CH_3CN$)

Additional info: The notes also reference recommended textbook problems for practice, which are useful for reinforcing these concepts.