Unit3) SYNTHESIS ) alkenes+ mechanism of electrophilic addition reactions

Question 1

what are alkenes

- what does the double bond in alkenes consist of

Answer 1

• unsaturated hydrocarbons containing at least one c-c double bond
  double bond in alkenes consists of both pi and sigma bond

Question 2

what accounts for the chemical reactivity of alkenes

Answer 2

the chemical reactivity of alkenes is due to the pi electrons being open to attack from electrophiles

Question 3

• how are alkenes obtained industrially

- what are alkenes used for

Answer 3

• cracking the naphtha fraction from crude oil
• cracking corresponding alkane
• used as feedstock for other chemicals , such as pharmaceuticals and polymers

Question 4

how can alkenes be prepared

Answer 4

• dehydration of alcohols

- base induced elimination of hydrogen halides from monohaloalkanes.

Question 5

preparation of alkenes : dehydration of alcohols

• what happens during dehydration
• what catalyst is used
• give example

Answer 5

• during dehydration of alcohol , the OH group is removed , and a H group on the adjacent c atom. alkenes + water is formed
• catalyst : Concentrated phosphoric acid , or concentration sulfuric acid
• butan 2 ol can form but 1 ene and but 2 ene

Question 6

preparation of alkenes : base induced elimination of hydrogen halides from monohaloalkanes

• what happens
• what catalyst and conditions used
• give example

Answer 6

• the OH ion from the base attacks the H atome next to the halogen bearing C atom. this forms a OH-H molecule. at the same time the pair of electrons in the C-H bond moves between the C-C forming a double bond. then the C-Br breaks heterolytically , releasing Br ion. ( the haloalkane looses a halogen and hydrogen atom)
• solvent used must be hot ethanol , so elimination is promoted over nucleophilic substitution. potassium hydroxide is dissolved in the ethanol.
• 2 bromopropane forms propene

Question 7

reactions of alkenes

Answer 7

alkenes undergo electrophilic addition reactions with :

• hydrogen to form alkanes - hydrogenation
• halogen to form dihaloalkanes - halogenation
• hydrogen halide to form monohaloalkanes - hydrohalogenation
• water to form alcohols - hydration

Question 8

hydrogenation

• what is it
• what is it catalysed by
• give example

Answer 8

• adding hydrogens to alkenes to form alkanes
• catalysed by nickel or platinum
• ethene reacts with hydrogen to form ethane

Question 9

halogenation

• what is it
• give example

Answer 9

• adding halogen to alkene to form dihaloalkane

- reacting bromine with ethene will form 1,2 dibromoethane

Question 10

hydrohalogenation

• what is it
• give example

Answer 10

• adding a hydrogen halid to alkene to form monohaloalkane

- eg reacting hydrogen bromide with but2ene it produces 2bromobutane

Question 11

hydration

• what is it
• what catalyst is used
• give an example

Answer 11

• addition of water to alkene across the double bond to form an alcohol
• catalysed by phosphoric acid
• for example hydration of but2ene forms butan2ol

Question 12

when a hydrogen halide (H-Br) or water (H-OH) is added to an unsymmetrical alkene(*) , how many products are formed

Answer 12

2 products 
(*) an alkene where the groups attached to one carbon of the double bond are not identical to the groups attached to the other carbon atom.

Question 13

give an example of addition of hydrogen halide to an unsymmetrical alkene - and what products are formed

Answer 13

• when H-Cl is added to but-1ene , 2chlorobutane and 1chlorobutane are both formed

Question 14

how can the major product of this reaction be predicted - markovnikovs rule

Answer 14

rule : states when H-X or H-OH is added to an unsymmetrical alkene , the major product out of the two is the one where the H atom ends up added to the C atom with the most H atoms already attached
- so the major product is 2chlorobutane

Question 15

halogenation steps mechanism (use bromine as the halogen)

- in each step what acts as the electrophile and what acts as the nucleophile

Answer 15

• as bromine approaches double bond,it becomes polarised. The double bond pushes the electrons in the bromine molecule towards the bromine furthest from the double bond, leaving the other br with a slight positive charge
• the slight positive br attacks the ethene molecule , forming a cyclic ion intermediate and a Br- ion.
• in this first step- the br is electrophile, and ethene is the nucleophile
• in step two, the br- ion attacks the cyclic ion intermediate from the side opposite to where the br atom is attached ( because the br atom in the intermediate is large and so prevents access to that side of the cyclic ion intermediate)
• in this second step the Br- ion is acting as a nucleophile , whilst the cyclic ion intermediate is acting as an electrophile

Question 16

hydrohalogenation mechanism

Answer 16

• H-Br molecule is the electrophile and is already polarised. Its H atom attacks the double bond in propene forming a carbocation intermediate. at the same time the h-br bond breaks heterolytically, and a br- ion is generated
• step 2 involves nucleophilic attack of the br ion on the carbocation. (unlike in the halogenation reaction, the br can attack from either side of the carbocation

Question 17

inductive stabilisation of carbocation intermediate in hydrohalogenation

Answer 17

-look in bright red page 64-65

Question 18

carbocations are _____

Answer 18

positive

Question 19

acid catalysed hydration mechanism steps

Answer 19

step 1 -
- hydronium ion of the catalyst is an electrophile and attacks the electron rich double bond in the propene molecule to form a carbocation

step 2
- carbocation undergoes rapid nucleophilic attack by a water molecule to give a protonated alcohol

step 3
- the protonated propan 2 ol is a strong acid and readily loses a proton to give the final product