ALKENES

What’re alkenes?

• unsaturated hydrocarbons

• general formula is CnH2n

• arrangement of bonds around the C=C is planar and has a bond angle of 120

What type of bonds does the C=C double covalent bond contain?

• one sigma bond

• one pi bond

• pi bonds are exposed and have high electron density

• they’re vulnerable to attack by electrophiles

What is stereoisomerism?

• have the same structural formula but different spatial arrangement of atoms

• alkenes exhibit E-Z stereoisomerism due to restricted rotation about the C=C bond

When do stereoisomers arise?

• when theres restricted rotation about the C=C double bond

• when there are 2 different groups attached both ends of the double bonds

Why do alkenes attract electrophiles?

due to the double bond which has a high electron density

What occurs in the electrophilic addition reaction between bromine and ethene?

• forms a dihalogenoalkane

• reagent - bromine

• conditions - room temperature

What is the mechanism in the electrophilic addition reaction between ethene and bromine?

What is the electrophilic addition reaction between a hydrogen bromine and alkene?

• forms a halogenoalkane

• reagent - HCl or HBr

• conditions - room temperature

What is the mechanism of the electrophilic addition reaction between but-2-ene and HBr?

What is ‘Markownikoff’s rule’?

bromine will be added to the carbon with the fewest hydrogens attached to it

Why would one product be made more than another product in electrophilic addition reactions?

the carbocation intermediate is more stable because the methyl groups on either side of the positive carbon are electron releasing and reduce the charge on the ion which stabilises it

What is the order of stability for carbocations?

1. tertiary

2. secondary

3. primary

What is the major product in electrophilic addition of alkenes?

formed via the more stable carbocation intermediate

What’re the 2 steps in forming an acohol from an alkene?

• electrophilic addition to form an alkyl hydrogensulfate

• hydrolysis to form an alcohol

What occurs when forming an alkyl hydrogensulfate?

• reagents - concentrated H2SO4

• conditions - room temperature

• CH2=CH2 + H2SO4 → CH3CH2COSO2OH

What is the mechanism for electrophilic addition between propene and sulfuric acid?

What occurs during the hydrolysis of the alkyl hydrogensulfate?

• forms an alcohol

• reagents - water

• conditions - warm mixture

• CH3CH2OSO2OH + H2O → CH3CH2OH + H2SO4

What is the mechanism for the hydrolysis of propyl hydrogensulfate and water?

What is H2SO4 displayed formula?

What is hydrolysis?

a reaction where the molecule is split by the addition of water

How are alkenes converted into alcohols in the industry?

• reacted with water in the presence of an acid catalyst

• high temperature of 300 to 600

• high pressure of 70 atm

• catalyst of concentrated H3PO4

How can you test for alkenes with bromine water?

• decolourises in the presence of a double bond

• turns from orange to colourless

How are addition polymers formed?

from alkenes

What is one repeating unit?

What is a repeating unit?

What is poly(chloroethene)?

• polymer

• waterproof, an electrical insulator, doesn’t react with acids

• pure form is a rigid plastic due to strong intermolecular bonding

• as a plasticiser its intermolecular forces are weakened, allowing the chains to move more easily