S3.2.2 Functional groups Notes

Functional Groups: The Building Blocks of Organic Chemistry

What Are Functional Groups?

1. For example, the hydroxyl group (–OH) in alcohols makes these compounds polar and capable of forming hydrogen bonds.
2. This influences their solubility in water and boiling points, which are critical properties in both natural processes and industrial applications.

Key Functional Groups and Their Properties

1. Let’s examine some of the most important functional groups in organic chemistry.
2. Each group has distinct properties and plays a critical role in determining the behavior of the molecules they belong to.

Halogeno Functional Group (–X)

1. Structure: –X, where X is a halogen (Cl, Br, or I).
2. Class: Halogenoalkanes (also called alkyl halides).
3. Properties:
   1. Contain a polar carbon–halogen bond.
   2. Reactivity depends on the halogen type; for example, the C–I bond is weaker and more reactive than the C–Cl bond.
4. Example: Chloroethane ($CH_3CH_2Cl$).

Hydroxy Group (–OH)

1. Structure: –OH.
2. Class: Alcohols.
3. Properties:
   1. Highly polar, enabling hydrogen bonding with other molecules.
   2. Increases solubility in water and raises boiling points.
4. Example: Ethanol ($CH_3CH_2OH$).

Carbonyl Group (C=O)

1. Structure: A carbon atom double-bonded to an oxygen atom.
2. Classes:
   1. Aldehydes: The carbonyl group is at the end of the carbon chain (e.g., ethanal, $CH_3CHO$).
   2. Ketones: The carbonyl group is within the carbon chain (e.g., propanone, $CH_3COCH_3$).
3. Properties:
   1. Polar, making aldehydes and ketones soluble in polar solvents.
   2. Reactivity depends on the surrounding groups.

Carboxyl Group (–COOH)

1. Structure: –COOH.
2. Class: Carboxylic acids.
3. Properties:
   1. Strongly polar and acidic due to the hydrogen atom in the –OH group.
   2. Forms hydrogen bonds, leading to high boiling points.
4. Example: Ethanoic acid ($CH_3COOH$).

Amino Group ($–NH_2$)

1. Structure: –$NH_2$.
2. Class: Amines.
3. Properties:
   1. Acts as a weak base.
   2. Forms hydrogen bonds, influencing solubility and boiling points.
4. Example: Methylamine ($CH_3NH_2$).

Amido Group ($–CONH_2$)

1. Structure: –$CONH_2$.
2. Class: Amides.
3. Properties:
   1. Highly polar due to the combination of carbonyl and amino groups.
   2. Commonly found in proteins as peptide bonds.
4. Example: Ethanamide ($CH_3CONH_2$).

Ester Group (–COOR)

1. Structure: –COOR, where R is an alkyl group.
2. Class: Esters.
3. Properties:
   1. Known for their pleasant, fruity odors.
   2. Less polar than carboxylic acids.
4. Example: Ethyl ethanoate ($CH_3COOCH_2CH_3$).

Phenyl Group ($C_6H_5$)

1. Structure: A benzene ring ($C_6H_6$) attached to a chain.
2. Class: Aromatics.
3. Properties:
   1. Stable due to delocalized $\pi$-electrons in the ring.
   2. Nonpolar, which affects solubility in water.
4. Example: Methylbenzene (toluene, $C_6H_5CH_3$).

Alkoxy Group (–OR)

1. Structure: –OR, where R is an alkyl group.
2. Class: Ethers
3. Properties:
   1. Ethers contain an oxygen atom bonded to two alkyl groups.
   2. Generally less reactive.
   3. Ethers do not form hydrogen bonds between molecules but can act as hydrogen bond acceptors due to the oxygen's lone pairs.
4. Example: Dimethyl ether ($CH_3OCH_3$)

Saturated vs. Unsaturated Compounds

Another way to classify organic compounds is by the type of bonds between carbon atoms.

Saturated Compounds

1. Definition: Contain only single bonds between carbon atoms.
2. Example: Alkanes (e.g., methane, $CH_4$).
3. Properties:
   1. Less reactive.
   2. Tend to undergo substitution reactions.

Unsaturated Compounds

1. Definition: Contain one or more double or triple bonds between carbon atoms.
2. Examples:
   1. Alkenes (e.g., ethene, $CH2=CH2$): Contain C=C double bonds.
   2. Alkynes (e.g., ethyne, $CH≡CH$): Contain C≡C triple bonds.
3. Properties:
   1. More reactive than saturated compounds due to $\pi$-bonds.
   2. Undergo addition reactions.