4.1.3 alkenes

Question 1

General formula

Answer 1

CnH2n

Question 2

Structure

Answer 2

C=C double covalent bond has one sigma and one pi bond

Pi bonds are exposed and have high electron density

Electrophiles are attracted to them

Question 3

Formation of pi bond

Answer 3

Sideways overlap of two p orbitals on each atom

Forms a pi bond above and below plane of molecule

Pi bond weaker than sigma

Question 4

Alkenes > alkane

Answer 4

Alkene > alkane

Reagent: Hydrogen

Nickel catalyst

Addition reaction

Question 5

Alkenes + bromine/chlorine

Answer 5

Alkene > dihaloalkane

Reagent : bromine/chlorine

Conditions: room temperature

Electrophilic addition

Heterocyclic bond fission

Question 6

Alkenes + hydrogen bromide

Answer 6

Alkene > haloalkane

Reagents: HCl or HBr

Conditions : room temperature

Electrophilic addition

H+ dipole attracted to electron-rich pi bond , Br more electronegative so polar HBr

Question 7

Alkenes > alcohol

Answer 7

Alkene > alcohol

Reagents: steam

High temperature (300-600C)

High pressure 70atm

Catalyst of concentrated H3PO4

Aka hydration

Question 8

Industrial importance of alkenes

Answer 8

Formation of polymers from ethene based monomers

Manufacture of margarine by catalytic hydrogenation of unsaturated vegetable oils using H2 and nickel catalyst

Question 9

Dealing with water polymers

Answer 9

Separation and recycling
- waste sorted then each type can be recycled by melting and moulding

Feedstock for cracking
- waste polymers used as feedstock for cracking process allowing for production of new plastics

Combustion for energy production
- incinerated and heat released used to generate electricity
- combustion of halogenated plastics (PVC) can lead to formation of toxic waste products e.g. HCl

Question 10

electrophile

Answer 10

electron pair acceptor

Question 11

nucleophile

Answer 11

electron pair donor

Question 12

Formation of sigma

Answer 12

two sp2 orbitals (one from each carbon) overlap to form a single C-C bond called a sigma bond

Question 13

stereoisomers

Answer 13

same structural formulae but different spatial arrangement of atoms

Question 14

E-Z isomers

Answer 14

exist due to restricted rotation about C=C bond

also arise when 2 different groups/atoms attached both ends of the double bond

Question 15

naming E-Z stereoisomers

Answer 15

determine priority groups on both sides of the double bond (atom with bigger atomic number)

if priority group is on same side of double bond it is labelled Z, if not then E

Question 16

Z stereoisomers

Answer 16

higher boiling point

polar molecule : polar bonds on same side of molecule so one side of the molecule is slightly negative

london forces and permanent dipole-dipole interactions

Question 17

E stereoisomers

Answer 17

lower boiling point

non-polar: polar bonds on opposite sides of the molecule so dipoles cancel out

only london forces so lower BP

Question 18

Markownikoff’s rule

Answer 18

bromine will be added to carbon with fewest hydrogens attached to it > major product formed via more stable carbocation intermediate
- tertiary > secondary > primary

major & minor products