alkenes

Question 1

what shape do alkenes have

Answer 1

trigonal planar (120 bond angle)

Question 2

what is a single bond and a double bond composed of

Answer 2

C−C single bond σ

C=C double bond σ and π

Question 3

Why is there no rotation around a C=C double bond?

Answer 3

because too much energy is required to rotate the double bond

Question 4

how are alkenes EZ isomers

Answer 4

because of the lack of rotation around the C=C bond

Question 5

what is an electrophile

Answer 5

An electron-pair acceptor

Question 6

what are the 3 types of electrophiles

Answer 6

· Hydrogen halides H-Cl H-Br H-I

· Halogens Cl-Cl Br-Br

· Sulphuric acid H-OSO3H

Question 7

give the reagent and product from Electrophilic Addition with Hydrogen Halides

Answer 7

· Reagent: HCl, HBr

· Type of product: Haloalkane

Question 8

What are alkyl groups?

Answer 8

Methyl (CH3), ethyl (CH2CH3), propyl (CH2CH2CH3) etc.

an alkane missing one hydrogen.

Question 9

What is a positive inductive effect?

Answer 9

The ability to release electron density through a covalent bond

Question 10

Why are tertiary carbocations more stable than primary carbocations?

Answer 10

tertiary carbocations have 3 alkyl groups bonded to a positive carbon whereas primary has only 1. There is a greater positive inductive effect from three alkyl groups in 3° carbocations compared to two alkyl groups in 2° carbocations

Question 11

in what types of alkenes do major and minor products form

Answer 11

only in asymmetrical

Question 12

Explain why 2-bromo propane is the major product when propene reacts with Hbr

Answer 12

· 2-bromopropane is made from a secondary carbocation

· 1-bromopropane is made from a primary carbocation

· Secondary carbocations are more stable than primary carbocations due to the positive inductive effect of the two alkyl groups (compared to one)

Question 13

give the reagent and product from Electrophilic Addition with Halogens

Answer 13

· Reagent: Cl2, Br2

· Type of product: Haloalkane

Question 14

Bromine reacts with alkenes, even though bromine is a non-polar molecule. Explain why bromine molecules react with the double bonds in alkenes.

Answer 14

· The C=C bond is electron rich

· and induces a dipole in the Br2

· δ+ Br attracted to C=C double bond

Question 15

what is the chemical test for alkenes. give observations when alkene is present and not present

Answer 15

· Reagent used to test for alkenes: Br2 (bromine liquid)

· Observation when alkene present: Orange to colourless

· Observation when alkene not present: No visible change

Question 16

give the reagent and product from Electrophilic Addition with Sulphuric Acid

Answer 16

· Reagent: H2SO4

· Type of product: Alkyl hydrogen sulphate

Question 17

describe the 3 step process to make alcohols from alkenes

Answer 17

Step 1

· alkene + H2SO4 → alkyl hydrogen sulphate

Step 2

· alkyl hydrogen sulphate + H2O → alcohol + H2SO4

Question 18

Role of the H2SO4 in the two-step transition from alkene to alcohol

Answer 18

· H2SO4 is reproduced (reformed) so it is a catalyst

Question 19

give the reagents conditions and product formed from direct hydration

Answer 19

· Reagent: H2O

· Conditions: Conc. H3PO4 catalyst / conc H2SO4

· Type of product: Alcohol

Question 20

how is an addition polymer formed

Answer 20

long chain made from lots of monomers (alkenes)

Question 21

why does PVC tend to be hard ad rigid

Answer 21

bc there are permanent dipole dipole forces between chains due to delta + c and delta - cl

Question 22

what does adding plasticisers to PVC’s do

Answer 22

reduce the effectiveness of attractions and makes plastic more flexible

Question 23

why are most plastics non- biodegradable

Answer 23

due to the strong covalent bonds present between the atoms in the polymer molecules