Organic chemistry: homologous series and functional groups

More than ninety per cent of all known compounds contain carbon: the fuels in cars and kitchens, plastics, medicines, soap, palm oil and every molecule in your body. Organic chemistry is the chemistry of carbon compounds. Its huge size becomes manageable once you learn to sort compounds into families called homologous series, each defined by a functional group.

Learning objectives

By the end of this lesson you should be able to:

• define organic chemistry and name the main sources of organic compounds
• explain why carbon forms so many compounds
• define a homologous series and state its characteristics
• identify the functional groups of the main homologous series
• distinguish molecular, empirical, structural and displayed formulas
• distinguish saturated and unsaturated, aliphatic and aromatic compounds

What organic chemistry is

Sources of organic compounds:

• petroleum (crude oil) and natural gas: the main source today
• coal
• plants and animals: oils and fats (palm oil), sugar, starch, cellulose, proteins, rubber
• synthesis in laboratories and factories

Why carbon forms so many compounds

1. Tetravalency: carbon has four outer electrons and forms four covalent bonds.
2. Catenation: carbon atoms join to each other in long chains, branches and rings, more than any other element.
3. Multiple bonds: carbon forms single, double and triple bonds (C–C, C=C, C≡C).
4. Bonds with other elements: strong bonds with hydrogen, oxygen, nitrogen, halogens and sulphur.
5. Isomerism: the same formula can have several different structures.

Homologous series

Characteristics of a homologous series:

1. All members have the same general formula (e.g. alkanes CₙH₂ₙ₊₂).
2. Successive members differ by –CH₂– (relative molecular mass 14).
3. They have the same functional group, so similar chemical properties.
4. Physical properties change gradually (boiling point and density rise with chain length).
5. They can be prepared by similar methods.

Figure 1. The first member of six homologous series. WASSCE Tutor diagram.

Types of formula

Classifying organic compounds

• Aliphatic compounds have open chains (alkanes, alkenes, alkanols).
• Aromatic compounds contain a benzene ring (C₆H₆), a stable ring of six carbon atoms. Benzene compounds are found in dyes, drugs and plastics.

Key points

• Organic chemistry is the chemistry of carbon compounds (except oxides, carbonates and cyanides).
• Carbon forms millions of compounds through tetravalency, catenation, multiple bonds and isomerism.
• A homologous series has the same general formula and functional group, members differing by CH₂, with similar chemical and gradually changing physical properties.
• Functional groups: C=C (alkenes), C≡C (alkynes), –OH (alkanols), –COOH (alkanoic acids), –COO– (esters).
• Saturated = single bonds only (substitution); unsaturated = C=C or C≡C (addition).

WASSCE examination focus

Quick check

1. State three characteristics of a homologous series.
2. What is the functional group of the alkanols?
3. Write the general formula of the alkenes.
4. Distinguish saturated from unsaturated hydrocarbons.

Answers:

1. Any three: same general formula; same functional group; successive members differ by CH₂; similar chemical properties; gradual change in physical properties; similar methods of preparation.
2. The hydroxyl group, –OH.
3. CₙH₂ₙ.
4. Saturated hydrocarbons have only single C–C bonds; unsaturated ones contain at least one C=C or C≡C bond.