4.2 hydrocarbons

Question 1

properties of alkanes

Answer 1

• saturated hydrocarbons
• CnH2n+2
• bonding = overlapping of orbitals (sigma bonds)

Question 2

boiling point of alkanes as length of chain increases

Answer 2

increases, as more points of contact, more london forces between compound, more energy needed

Question 3

boiling point of non-branched vs branched alkanes

Answer 3

boiling point of branched alkanes = lower as they have fewer points of contact = less london forces = less energy required

Question 4

how reactive are alkanes + why

Answer 4

low
* high bond enthalpy of bonds
* lack of polarity (C-C bonds are non-polar)
* C-H act as if non-polar

Question 5

products of complete combustion of alkanes

Answer 5

• carbon dioxide
• water

Question 6

products of incomplete combustion of alkanes

Answer 6

• carbon monoxide
• carbon particles
• carbon dioxide
• water

Question 7

reactions of alkanes with halogens

Answer 7

radical substitution
1. initiation
2. propogation
3. termination

Question 8

initiation stage

Answer 8

1. UV light breaks Cl-Cl bond by homolytic fission
2. forms 2 Cl radicals

Question 9

propogation stage:

Answer 9

1. Cl radicals remove hydrogen from alkane to form HCl and alkyl radical
2. alkyl radical reacts with other Cl2
3. alkyl+Cl and Cl radical formed

Question 10

termination stage

Answer 10

1. removes all radicals by 3 reactions taking place

Question 11

properties of alkenes

Answer 11

• unsaturated
• CnH2n
• double bond is made up of a sigma and pi bond

Question 12

sigma bond

Answer 12

overlapping of orbitals directly between atoms

Question 13

pi bond

Answer 13

overlapping of p-orbitals above and below carbon atoms

Question 14

shape of alkenes

Answer 14

trigonal planar - 120º

Question 15

shape of alkanes

Answer 15

tetrahedral

Question 16

stereoisomers

Answer 16

compounds with the same structural formula but atoms are arranged differently in space

Question 17

two types of isomerism

Answer 17

1. E-Z isomerism
2. optical isomerism

Question 18

why does E-Z isomerism occur

Answer 18

restricted rotation around double bonds when there are two different groups attatched to each carbon around a double bond

Question 19

E-isomers

Answer 19

have high priority groups on opposite sides

Question 20

Z-isomers

Answer 20

have both high priority groups on the same side

Question 21

cis-trans isomerism

Answer 21

special case of E-Z when
* two of the groups attached to each C is the same
* H atom is attached to each C

Question 22

cahn-ingold-prelog rules

Answer 22

if 4 different groups attatched to C=C:
* atoms with highest mass = highest priority
* same side = Z
* different sides = E

if the atoms bonded directly have the same mass, the atoms next to them are examined

Question 23

addition of hydrogen to alkenes

Answer 23

hydrogenation
double bond removed by adding H2 with a nickel catalst at 150º

Question 24

addition of halogens to alkenes

Answer 24

add diatomic halogen to determine if double bond present. bromine water added, solution decolorises and dibromoalkane formed

Question 25

addition of steam to alkenes

Answer 25

water acts as H and hydroxyl group to from two different alcohols in presence of H3PO4

Question 26

what is an electrophile

Answer 26

electron pair acceptor

Question 27

electrophilic addition of hydrogen bromide and an alkene

Answer 27

1. slightly positive H is attacked by the double bond 2. pi bond breaks and bond forms between C and H 3. bond in H-Br breaks by heterolytic fission 4. two ions are attracted to complete reaction

Question 28

electrophilic addition of bromine and an alkene

Answer 28

1. induced dipole between Br2 2. attacked by double bond 3. carbocation formed 4. carbcation reacts with bromide ion to form product