2.3 Alkenes

What are the prefixes for the alkenes from 2 to 8 carbons?

Eth- (2), Prop- (3), But- (4), Pent- (5), Hex- (6), Hept- (7), and Oct- (8). (There is no 1-carbon alkene).

What is a functional group?

The part of a molecule that gives it its specific chemical properties and allows it to take part in reactions.

How do you name an alkene when the double bond can be in different positions?

Number the carbon chain from the end that gives the carbon-to-carbon double bond the lowest possible number, and put that number before "-ene" (e.g., but-2-ene).

What is the general formula for alkenes?

CnH2n (where n is the number of carbon atoms).

How do you work out the number of hydrogen atoms in any alkene?

Take the number of carbons and double it.

How do you work out the number of carbon atoms in an alkene if you are given the hydrogens?

Take the number of hydrogens and divide by 2.

Why are alkenes able to undergo addition reactions?

They are unsaturated and contain a reactive carbon-to-carbon double bond that can open up to allow atoms to add across it.

What is chlorination?

An addition reaction where chlorine (Cl2) adds across a carbon-to-carbon double bond to form a saturated dichloroalkane (e.g., ethene + chlorine -> 1,2-dichloroethane).

What is hydration?

An addition reaction where water (H2O) adds across a carbon-to-carbon double bond to form an alcohol (e.g., ethene + water -> ethanol).

Why can an addition reaction with an unsymmetrical alkene (like pent-2-ene) form two different products?

Because the two halves of the molecule being added (e.g., H and OH from water) can attach to either of the two carbons that shared the double bond (e.g., forming pentan-2-ol or pentan-3-ol).

What is the chemical test for unsaturation?

Add bromine water (or bromine solution) to the hydrocarbon and shake.

What happens when bromine water is added to an alkene (unsaturated)?

The bromine water rapidly decolourises (turns from orange/brown to colourless) because an addition reaction takes place.

What happens when bromine water is added to an alkane or cycloalkane (saturated)?

There is no colour change (the solution stays orange/brown) because they have no double bond and cannot undergo an addition reaction.