3.4.2 addition reactions of alkenes

electrophile

• an electron pair acceptor

• a positive/electron deficient species which is attracted to a region of high electron density

addition reaction

• a reaction that converts a double bond into a single bond.

• a reaction that increases the number of substituents.

• when two reactants form only one product.

electrophilic addition

• an electrophile is attracted to the high electron density of C=C in alkenes.

• the molecule adds across the double bond, meaning the atoms of the molecule end up on two adjacent C atoms.

if an alkene has only one double bond, give the type of product formed in the electrophilic addition reactions of that alkene with:

1. aqueous Br₂

2. cold/room temp HBr

3. cold/room temp concentrated H₂SO₄

4. H₂O as steam (hydration)

1. dibromoalkane

2. bromoalkane

3. alkyl hydrogensulfate

4. alcohol

how does Br₂ act as an electrophile?

• Br₂ is non-polar.

• the high electron density of the C=C bond attracts the Br₂ molecule and repels the electron pair of the Br-Br bond.

• Br-Br becomes polarised and now has an induced dipole.

• Br^δ+ acts as an electrophile and is attracted to C=C, and it accepts the electron pair in the pi bond of the C=C bond.

how do HBr and H₂SO₄ act as electrophiles?

for H-Br:

• H^δ+ acts as the electrophile, :Br^- is formed.

for H - OSO₂OH

• H^δ+ acts as the electrophile, :O^-SO₂OH formed (HO-O₂S-O^-, hydrogen sulfate ion)

how does H₂O act as an electrophile?

• H⁺ is a better electrophile as it is fully positive so has a greater attraction to the high electron density of the C=C bond, unlike water which is neutral.

• in the hydration of alkenes, H⁺ from phosphoric/sulfuric acid catalyst acts as an electrophile, and bonds to a C of C=C.

• a lone pair on the water molecule later joins to C⁺ of carbocation formed.

how can bromine water be used to test for unsaturation in alkenes?

you can test for the double bond:

• gently shake alkene with bromine water.

• positive result: goes from orange-brown to colourless, decolourises.

• electrophilic addition occurs.

• one Br₂ molecule adds across one double bond in the alkene.

carbocation

• a molecule containing a C atom with a positive charge.

• is an intermediate in electrophilic addition reactions of alkenes.

how can there be two possible products in electrophilic addition reactions?

• using a symmetrical alkene makes only one possible product.

• using an unsymmetrical alkene makes a major and minor product, two position isomers.

  • the isomer formed depends on which C atom is + in the carbocation intermediate.

  • the major/minor product can exist as a pair of enantiomers.

• exception: adding a halogen molecule to an unsymmetrical alkene makes only one product.

what is the difference between primary, secondary and tertiary carbocations?

• primary: the positive C atom is attached to only 1 other C atom.

• secondary: the positive C atom is attached to 2 other C atoms.

• tertiary: the positive C atom is attached to 3 other C atoms.

how are major and minor products formed in electrophilic addition reactions of unsymmetrical alkenes?

• major products are formed via the more stable carbocation.

• minor products are formed via the least stable carbocation.

what type of carbocation is the most stable and why?

• most stable: tertiary.

• positive inductive effect: electrons in alkyl groups shift towards the positive charge of the carbocation, to stabilise the positive charge.

• tertiary carbocations are more stable, as there is a greater positive inductive effect from more electron-releasing alkyl groups stabilising the positive charge.

• so, the carbocation exists for longer so is more likely to form its product.

Markovnikov’s rule

when a hydrogen halide reacts with an unsymmetrical alkene, the hydrogen is more likely to bond to the carbon which is already attached to the greater number of hydrogen atoms

• this will form a more stable carbocation so will form the major product

conditions for the hydration reaction to produce ethanol from ethene

• water in the form of steam

• concentrated phosphoric acid catalyst

• temperature of 300°C

• pressure of 60 - 70 atm

reaction of alkylhydrogensulfate and water

alkylhydrogensulfate + water → alcohol + sulfuric acid

the OH of the water molecule replaces the hydrogen sulfate