3.2 Hydrocarbons

Question 1

Alkenes

Answer 1

Alkenes are unsaturated hydrocarbons with a double covalent bond between two carbon atoms. Alkenes have the general formula CnH2n.

Question 2

fractional distillation

Answer 2

separates hydrocarbons within crude oil

separates the hydrocarbons into simpler mixtures depending upon their boiling points.

hydrocarbon fractions from the primary distillation of crude oil are of limited use without further processes

Question 3

Alkanes

Answer 3

Alkanes are saturated hydrocarbons with single covalent bonds between atoms. Alkanes have the general formula CnH2n+2.

sigma bonds σ

Question 4

cracking

Answer 4

Cracking produces smaller alkanes and unsaturated alkenes, such as ethene and propene, which are the basis of the manufacture of many polymers.

large molecules to smaller more useful molecules

Question 5

conbustion of Alkanes

Answer 5

good fuels

release lots of energy when they burn

complete or incomplete combustion

complete - produces carbon dioxide and water

Question 6

Incomplete combustion

Answer 6

reaction where the oxygen supply is limited.

As a result, the products contain less oxygen atoms than they might have had with complete combustion.

This means carbon monoxide or even just carbon can be a product rather than carbon dioxide.

Question 7

drawback of fossil fuel combustion

Answer 7

CO2 production

incomplete combustion - formation of soot (carbon) and toxic carbon monoxide

combustion of impurities in the fuels - Acidic gases, such as sulfur dioxide (SO2) and toxic nitrogen oxides (NOx)

Question 8

fission

Answer 8

breaking of a covalent bond

Question 9

homolytic fission

Answer 9

The covalent bond breaks with both atoms receiving one electron.

forms free radicals - species with an unpaired electron, which makes it very reactive.

For example, Chlorine Cl2 breaks to form two chlorine radicals, shown as Cl●.

Question 10

hetrolytic fission

Answer 10

The covalent bond breaks with one atom retaining both electrons.

This forms positive and negative ions.

For example, HCl breaks to form H+ and Cl- ions.

Question 11

photochloronation reactions of alkanes

Answer 11

chain reactions

producing chloroalkanes

reaction mechanism is a free radical substitution

Reagent: Cl2

Conditions: UV light

The reactions involve the substitution of hydrogen atoms in the alkanes for chlorine atoms.

Question 12

free radical substitution

Answer 12

Homolytic fission of halogen bond in presence of UV light

produce halogenalkanes

Question 13

What happens to the bonds in free radical substitution

Answer 13

UV light breaks halogen bonds

Producing intermediates (molecules formed during a reaction)

called free radicals

Question 14

what do free radicals do to alkanes

Answer 14

attack alkanes

lead to reactions:

initiation

propagation

termination

Question 15

initiation

Answer 15

halogen broken down with UV light

the formation of radicals by homolytic fission of the chlorine bond. UV light is necessary for this stage.

Question 16

propagation

Answer 16

hydrogen replaced with halogen radical formed

radical acts as a catalyst

reactions take place to form further radicals and some stable products, such as chloromethane and dichloromethane

Question 17

termination

Answer 17

two radicals joing to end chain reaction

form one stable product

Question 18

propagation extra

Answer 18

can result in multiple substitutions

chain reaction

conditions altered to favour terimnation step

Question 19

alkenes

Answer 19

unsaturated hydrocarbons

carbon carbon double bond

area of high electron density - susceptible to attack from electrophiles

double bond has σ and π

Question 20

bonds in alkenes

Answer 20

Question 21

tests for alkenes

Answer 21

bromine water - orange/brown to colourless forms dibromo-alkane, double bond opens up forms bonds with bromine atoms

potassium maganate (VII) - under acidic conditions purple to colourless, alkaline conditions purple to dark green

Question 22

Stereoisomers

Answer 22

same molecular formula

different spatial arrangement

Question 23

types of stereoisomers

Answer 23

E-isomer - high priority group diagonally across

Z-isomer - high priority group same side

Question 24

carbocations

Answer 24

primary carbocation

secondary carbocation

tertiary cation

reaction proceeds by most stable intermediate

in addition reactions multiple products can form

Question 25

Sigma bond σ

Answer 25

S orbitals overlap can rotate no change in molecule alkanes form only sigma bonds

Question 26

Pie bond π

Answer 26

alkenes between two atoms formed by side to side overlap of p orbitals. can't rotate, leads to goemetric isomers gives a region of high electron density This is susceptible to electrophilic attack/ attack by an electron deficient species * (This attack) leads to addition reactions

Question 27

just hydrocarbons

Answer 27

Hydrogen groups on same side = CIS isomer Hydrogen groups on opposite sides = TRANS isomer

Question 28

EG bonds can be broken hetrolytically

Answer 28

hetrolytic fission

Question 29

electrophile

Answer 29

species attracted to an area of high electron density can accept a pair of e- may or may not be charged

Question 30

electrophilic addition

Answer 30

happens to alkenes due to electron dense double bond electrophiles attack double bond in alkenes lead to formation of positive carbocation intermediate

Question 31

electrophilic addition of bromine ethene

Answer 31

Question 32

asymetric alkene

Answer 32

groups or atoms attached to either end of the carbon-carbon double bond are different.

Question 33

reaction of asymetric alkenes with H-BR

Answer 33

Question 34

reaction of an asymmetric alkene with H-Br

Answer 34

Question 35

overall reaction free radical substitution

Answer 35

Question 36

overall reaction free radical substitution

Answer 36

Question 37

single covalent bonds rotation

Answer 37

can freely rotate atoms can move around the molecule Therefore, the positioning of the chlorine atoms around the carbon atoms in the diagram is not important.

Question 38

double carbon carbon bond isomérisation

Answer 38

no free rotation This causes E -Z isomerism (also known as geometric isomerism) in this molecule. therefore the positioning of the chlorine atoms does matter.

Question 39

E -Z isomerism (also known as geometric isomerism)

Answer 39

E - opposite sides Z - same side same structural formula but different spatial arrangement of groups around the carbon double bond. For a molecule to show E-Z isomerism, both carbon atoms in the double bond must have two different groups bonded to them.

Question 40

atom priority E Z isomerisation

Answer 40

The atom bonded to the carbon with the higher atomic number (smaller) is given the higher priority.

Question 41

2 geometric isomers of 1,2-dichloroethene

Answer 41

Question 42

single covalent bond formed by

Answer 42

the overlap of s orbitals or an s orbital with a p orbital, or the end to end overlap of two p orbitals. These are all forms of sigma σ bond and the electrons are between the two atoms.

Question 43

double covalent bond formed by

Answer 43

sigma bond and a pi bond. **The pi bond is the vertical overlap of two p orbitals**, with the electrons above and below the plane of the two carbon atoms.

Question 44

ethene pi and sigma bonds

Answer 44

Question 45

reactivity of alkenes

Answer 45

double bond - region of high electron density susceptible to electrophilic attack double bond is weaker than two single bond strengths therefore it breaks quite easily to form a single bond. This makes alkenes very reactive compared to alkanes and thus they easily undergo addition reactions such as hydrogenation, hydration and bromination.

Question 46

hydrogenation of alkenes Reagent conditions and eg

Answer 46

addition of hydrogen across the alkene double bond, for example, in turning polyunsaturated fats into saturated fats (liquid vegetable oils into solid edible fats or margarines). Reagent: Hydrogen H2(g) Conditions: Heat Catalyst: Nickel metal (in the form of fine grains known as Raney nickel) Type of reaction: Addition / hydrogenation For example, hydrogen can be added to either but-1-ene or but-2-ene to form butane. eg turning unsaturated oils into saturated fats

Question 47

electrophilic addition reactions of alkenes 1. addition of hydrogen bromide to alkenes Reagent reaction eg

Answer 47

1. Addition of hydrogen bromide to alkenes Reagent: Hydrogen bromide HBr Reaction mechanism: Electrophilic addition The electron-rich double bond is attracted to the delta positive hydrogen – this is called an electrophile. The double bond breaks to form the intermediate, known as a carbocation. The HBr bond also breaks, allowing the hydrogen to bond to the carbon and also forming a bromide ion. The bromide ion then joins onto the carbocation. Curly arrows show the movement of electron pairs. This reaction produces mostly 2-bromopropane (as shown above) rather than 1-bromopropane. This is due to the greater stability of the secondary carbocation compared to the primary carbocation.

Question 48

electrophilic addition reactions of alkenes 2. Addition of bromine to alkenes

Answer 48

Bromine, Br2, can also be added across an alkene double bond in a similar electrophilic addition reaction. For example, but-1-ene reacts with bromine to form 1,2-dibrombutane. C4H8 + Br2 --> C4H8Br2 The mechanism is similar, although the bromine bond is only polar once it is in the region of the carbon double bond.

Question 49

Alkenes can be joined together in long chains of thousands of carbon atoms.

Answer 49

polymer

Question 50

Polymerisation of ethene Conditions and catalyst

Answer 50

The polymerisation of ethene needs heat (200˚C), pressure (2000 atm) and an oxygen initiator. Ziegler catalysts can be used to create different polymer structures. The process is called addition polymerisation – this is because the carbon double bond is broken and the monomer units are added onto one another.

Question 51

polymerisation of propene will produce the repeating unit:

Answer 51