alkenes

Question 1

general formula

Answer 1

CnH2n

Question 2

what type of hydrocarbons are the alkenes

Answer 2

unsaturated because carbon atoms in c=c do not have the maximum number of hydrogen atom

Question 3

structure and reactivity:

Answer 3

the C=C double bond contains one sigma and one pi bond

the pi bond forms due to the overlap of p orbitals- this restricts rotation and gives rise to geometric isomerism

Question 4

why are the alkenes more reactive than alkanes

Answer 4

the C=C bopnd os basically 4 electreons fixed between carbon nuclei, this gives it a high electron density and is attacked by electrophiles

Question 5

what is an electrophile

Answer 5

an electrophile is a species which seeks out negative centres and accepts a pair of electrons to form a new covalent bond

Question 6

how does electrophilic addition occur

Answer 6

the pi bond in C=C is weak and takes only small amounts of energy to be broken

• this then allows two new sigma bonds to be formed, the energy released compensates for the breaking of the pi bond
• electrophilic addition is therefore exothermic

Question 7

examples of electrophilic addition

Answer 7

1) reactions with halogens to form dihaloalkanes
2) hydrogenation to form alkanes
3) hydration to form alcohols
4) reactions with hydrogen halides to form haloalkanes

Question 8

conditions of each specific reaction

Answer 8

1) with halogens- room temperature + aq solution
2) hydrogen halides- mix in the gas phase
3) with hydrogen- nickel catalyst, 100 degrees
4) with water- phosphoric acid (H3PO4) + 100 C

Question 9

explaining the electrophilic addition mechanism

Answer 9

1) as a halogen molecule approaches the electron rich C=C bond, it becomes polarised as electrons are repelled away
2) the halogen molecule (electrophile) then becomes polarised as it accepts the electron pair from the weak pi bonds of C=C to form new sigma bonds
3) at the same time, the halogen-halogen bonds also breaks forming a carbocation intermediate
4) lastly the CI reacts with the remaining halide ion to form a new product

Question 10

markownikoffs rule

Answer 10

when a hydrogen halide reacts with an unsymmetrical alkene, the hydrogen of the hydrogen halide attaches itself the carbon of the alkene

-bearing larger number of hydrogens and smaller number of carbons

Question 11

addition reactions of unsymmetrical alkenes produce

Answer 11

more than one product

Question 12

types of carbocations

Answer 12

primary- one carbon chains attached to C+

secondary- two carbon chains attached to C+

tertiary- three carbon chains attached to C+