Organic chemistry/Functional groups

Functional groups: they are what makes a compound a compound: they give a compound its character. There are two types of functional groups: carbon side-chains and non carbon groups.

Carbon Chains

Carbon chains are composed of carbon molecules, and these are named as according to their length:

Carbon chart

Carbon chain length	Name prefix
1	Meth-
2	Eth-
3	Prop-
4	But-
5	Pent-
6	Hex-

The carbon main chain nomenclature is covered in Organic Chemistry/Naming, and the same rules apply to side chains.

Functional Groups with Oxygen

Alcohols

General Alcohol Formula: C_nH_2n+1OH

Suffix: "ol"

Example: CH₃CH₂OH - Ethanol (IUPAC)

Example: CH₃CH₂OH - Ethyl Alcohol (Common Name)

Example: CH₃CH₂CH₂OH - 1-Propanol (IUPAC)

Example: CH₃CH₂CH₂OH - Propyl Alcohol (Common)

Example: CH₃CHOHCH₃ - Propan-2-ol (IUPAC)

Example: CH₃CHOHCH₃ - Isopropyl Alcohol (Common)

The numbered prefix is indicative of an isomer, not the location of a double bond.

Ketones

Ketones are carbonyl compounds. Thus they contain the C=O group. This group is responsible for the typical peak present in the mass spectrometer profiles for carbonyl compounds. The suffix "-one" is added to the alkane name to indicate a ketone. Ketones, if formed from alcohols, must be formed from secondary alcohols as you need to have two groups either side of the "carbonyl" carbon.

General Ketone Formula: C_nH_(2(n-1)+2)O

Suffix: "one"

Examples of ketones include:

CH₃COCH₃ Name: propanone. This is the lowest member of the series.

CH₃COCH₂CH₃ Name: butanone.

CH₃CH₂COCH₂CH₃ Name: pentan-3-one. The "-3-" indicates the position of the C=O (carbonyl) group. It shows that the carbonyl group is present on the third carbon atom in the chain. Such naming is essential in ketones (except propanone and butanone). For example, an isomer of pentan-3-one is pentan-2-one, which has a different structural formula.

Ethers

In Ethers the functional group is -O-. The General Formula of ethers is R-O-R or R-O-R'. R-O-R are simple ethers. Whereas R-O-R' represents mixed ethers. R and R' are different alkyl groups. According to IUPAC they are called Alkoxy Alkanes (O-R is alkoxy group).

For Example,

CH₃OCH₃ is a simple ether whose common name is Dimethyl Ether and the IUPAC name is Methoxy Methane.

C₂H₅OCH₃ is a mixed ether whose common name is Ethyl Methyl ether and the IUPAC name is Methoxy Ethane. You should name with the lowest group first: methyl, then ethyl, etc... which explains the IUPAC difference in naming, as the methyl come before the ethyl.

Esters

Carbon Compounds consisting of a carbonyl carbon adjacent to an ether (-COO-). Esters are often characterized by a unique smell or taste. Esters are used by manufacturers due to their unique smells and tastes. Suffix is -oate.

Aldehydes

Aldehydes are carbonyl compounds with a C=O and a H bonded to the same carbon. The suffix attached to an aldehyde is "al". The quickest way to form an aldehyde is to oxidise a primary alcohol using potassium dichromate.

Suffix: "al"

General formula: C_nH2_n+1CHO

Functional Group: CHO

Examples of aldehydes:

CH₂O Formaldehyde (common name) or Methanal (IUPAC), is the simplest aldehyde.

CH₃CH₂CHO Propanal

CH₃CH₂CH₂CHO Butanal

Carboxylic Acids

Carboxylic acids are carbonyl compounds containing a C=O and an -OH group bonded to the same carbon. They can be obtained by oxidising a aldehyde or a primary alcohol using potassium dichromate (K2Cr2O7) with concentrated sulphuric acid.

Suffix: "oic acid"

General formula: C_nH_2n-1COOH

Functional Group: COOH

Examples of carboxylic acids:

CH₃COOH Ethanoic Acid (usually called Acetic acid)

CH₃CH₂COOH Propanoic Acid

Benzene and phenols

Benzenes, part of the aromatic group, are all based on the benzene ring (C₆H₆). In it, there are three double bonds that create a resonance structure. Resonance theory states that we can draw two different structures for benzene where the double bonds are in different places, but neither structure adequately represent the molecule, so instead we think of the molecule as a resonance hybrid, a combination of the two. Benzene is incredibly stable, and is also planar. Although the term aromatic should refer to a smell, is really referring to a Huckel number of pi electrons.

General formula: C₆H₆

Derivatives of benzene are named by numbering the C atoms in a clockwise manner, starting at one for the first group. Alkyl monosubstituted benzenes are names as alkyl benzenes.

Example: C₆H₅CH₃ - Methylbenzene, or Toluene.

If the alkyl branch is attached to the benzene ring by a carbon other than an end carbon, the benzene is then considered a branch of the carbon chain, and is called phenyl.