Alkanes, alkenes, alcohols Notes

Introduction to Organic Chemistry

1. Organic chemistry is the study of carbon-based compounds, especially those containing C–C and C–H bonds.
2. Most molecules in living things – sugars, fats, proteins, DNA – are organic, as well as many fuels, plastics, medicines and dyes.
3. So when we study alkanes, alkenes and alcohols, we are really starting to explore the huge family of organic molecules built around carbon.

Why Carbon Is So Important

Carbon is special because it can:

1. Form four covalent bonds (it is tetravalent).
2. Bond to other carbon atoms to form long chains and rings.
3. Form single, double and triple bonds (C–C, C=C, C≡C).
4. Bond to many other elements: H, O, N, halogens, S, etc.

Homologous Series and Functional Groups

To organise this huge number of compounds, chemists use the ideas of:

1. Homologous series
2. Functional groups

Homologous Series

A homologous series is a family of organic compounds that:

1. Have the same functional group
2. Follow the same general formula
3. Differ by –CH₂– units between members
4. Show similar chemical properties
5. Show gradual trends in physical properties (e.g. boiling point increasing with chain length)

Functional Groups

1. A functional group is a specific group of atoms in a molecule that is responsible for its characteristic reactions.
2. Different functional groups → different types of reactions and uses.

Naming Organic Compounds: IUPAC Basics

1. In organic chemistry we use a systematic naming system called IUPAC nomenclature.
2. For now, we’ll focus on simple, unbranched alkanes, alkenes and alcohols.
3. You will build three key skills:
   1. Recognizing the root name (how many carbons)
   2. Identifying the functional group (what family)
   3. Using the correct suffix (-ane, -ene, -ol)

Root Names – Number of Carbon Atoms

The root tells you how many carbons are in the longest continuous chain:

For MYP level, 1–6 carbons is usually enough to start with.

Suffix – Which Functional Group?

The ending of the name (suffix) tells you which homologous series the compound belongs to:

1. Alkanes → -ane
   1. e.g. ethane, propane
2. Alkenes → -ene
   1. e.g. ethene, propene
3. Alcohols → -ol
   1. e.g. ethanol, propanol

Position Numbers (for alkenes and alcohols)

1. When the functional group can be in different positions along the chain, we add a number to show where it is.
2. Number the carbon chain so that the functional group gets the lowest possible number.

Types of Formula in Organic Chemistry

1. In organic chemistry, we often represent molecules in different ways depending on how much detail we need.
2. It’s important to recognise and move between these formula types.
3. We will mainly use:
   1. Molecular formula
   2. Structural (or condensed structural) formula
   3. Displayed formula

Molecular Formula

1. This shows the actual numbers of each type of atom in a molecule.
   1. Ethane: C₂H₆
   2. Ethene: C₂H₄
   3. Ethanol: C₂H₆O
2. It does not show how the atoms are connected.

Structural (Condensed) Formula

1. This shows how atoms are joined together, often grouping hydrogens with their carbon.
   1. Ethane: CH₃–CH₃
   2. Ethene: CH₂=CH₂
   3. Ethanol: CH₃CH₂OH or CH₃–CH₂–OH
2. This is the most common style for quick organic work.

Displayed Formula

1. This shows every bond and every atom.
2. For ethanol, you would show:
   1. Two carbon atoms bonded together (C–C)
   2. The first carbon with 3 hydrogens (CH₃–)
   3. The second carbon with 2 hydrogens and bonded to an O
   4. The oxygen bonded to H (–OH)
3. So the diagram shows all individual C–H, C–C, C–O and O–H bonds.

General Formulae and Functional Groups

As discussed earlier, organic compounds are grouped into homologous series: families of molecules that:

1. Share the same functional group
2. Follow a general formula
3. Differ by –CH₂– units
4. Have similar chemical properties and trends in physical properties

Further, we are going dive into more details about three important series: alkanes, alkenes and alcohols.

Alkanes – Saturated Hydrocarbons

General formula: $$C_nH_{2n+2}$$

Functional group:

1. Alkanes have no special functional group beyond the C–C and C–H single bonds.
2. They are often described simply as “saturated hydrocarbons”.

Industrial uses:

1. Methane → main component of natural gas; used as a fuel in power stations and homes.
2. Propane → used in LPG cylinders for cooking and heating.
3. Octane → part of petrol (gasoline) used as fuel for cars.

Alkenes – Unsaturated Hydrocarbons

General formula: $$C_nH_{2n}$$

Functional group:

1. C=C double bond.
2. The double bond consists of:
   1. One sigma (σ) bond (like in alkanes)
   2. One pi (π) bond, which is more exposed and more reactive

Industrial uses:

1. Ethene → used to make poly(ethene) (polythene), a common plastic.
2. Propene → used to make poly(propene), used in ropes, crates, and many plastic objects.

Alcohols – Hydroxyl Functional Group (-OH)

General formula (simple, primary alcohols): $$C_nH_{2n+1}OH$$

Functional group: –OH (hydroxyl group).

Uses:

1. Methanol → industrial solvent, feedstock for making other chemicals.
2. Ethanol → alcoholic drinks, fuel, solvents, hand sanitiser.
3. Propanol → cleaning agents, inks, and as a solvent.

Saturation vs Unsaturation – Structure and Reactivity

The idea of saturation is directly linked to the types of bonds between carbon atoms.

Saturated – Alkanes

1. Saturated compounds have only single bonds between carbon atoms.
2. Each carbon is bonded to as many hydrogens as possible.
3. Consequences for reactivity
   1. Alkanes are relatively unreactive.
   2. They mainly undergo combustion (burning) and substitution reactions (under special conditions, e.g. with halogens and UV light).
4. Test with bromine water
   1. Alkanes do not react with bromine water in normal conditions.
   2. Bromine water stays orange/brown.
   3. This makes alkanes a good “negative control” when testing for unsaturation.

Unsaturated – Alkenes

1. Unsaturated compounds contain C=C double bonds (or C≡C triple bonds, though those are alkynes).
2. This means there is room to add more atoms (usually hydrogen or halogens) across the double bond.
3. The π bond in the double bond:
   1. Is more exposed to attacking species.
   2. Makes alkenes more reactive than alkanes.

Addition Reactions of Alkenes

Because of the C=C bond, alkenes undergo addition reactions:

1. The double bond opens up.
2. New atoms are added to the two carbons that were double-bonded.
3. The product becomes saturated (like an alkane).

Alcohols – Structure, Properties and Uses

The presence of the –OH group in alcohols gives them distinct properties compared to alkanes and alkenes.

Solubility in Water

1. Short-chain alcohols (up to about propanol):
   1. Are very soluble (often miscible) in water.
   2. This is because the –OH group can form hydrogen bonds with water molecules.
2. As the carbon chain length increases:
   1. The non-polar hydrocarbon part becomes larger.
   2. The molecule behaves more like an alkane.
   3. Solubility in water decreases.

Boiling Points

Alcohols have higher boiling points than alkanes and alkenes with similar molar masses.

1. Reason:
   1. The –OH group allows hydrogen bonding between alcohol molecules.
   2. Hydrogen bonds are stronger intermolecular forces than the weak dispersion forces between simple hydrocarbons.
   3. More energy (higher temperature) is needed to separate alcohol molecules.
2. Trend
   1. As the carbon chain gets longer, boiling point increases (more electrons, stronger dispersion forces), even within alcohols.

Flammability and Use as Fuels

Alcohols are flammable and burn in air to form carbon dioxide and water.

Typical Uses Linked to Structure

Because of their hydroxyl group and polar nature, alcohols are:

1. Good solvents for both polar and some non-polar substances.
2. Good for disinfecting and cleaning because they can dissolve oils and disrupt cell membranes.