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2. Chemistry A-level > 3.2 Hydrocarbons > Flashcards

3.2 Hydrocarbons Flashcards

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1
Q

Alkanes

A

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

sigma bonds σ

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2
Q

Alkenes

A

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

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3
Q

fractional distillation

A

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

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4
Q

cracking

A

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

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5
Q

conbustion of Alkanes

A

good fuels

release lots of energy when they burn

complete or incomplete combustion

complete - produces carbon dioxide and water

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6
Q

Incomplete combustion

A

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.

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7
Q

drawback of fossil fuel combustion

A

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)

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8
Q

fission

A

breaking of a covalent bond

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9
Q

homolytic fission

A

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●.

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10
Q

hetrolytic fission

A

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.

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11
Q

photochloronation reactions of alkanes

A

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.

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12
Q

free radical substitution

A

Homolytic fission of halogen bond in presence of UV light

produce halogenalkanes

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13
Q

What happens to the bonds in free radical substitution

A

UV light breaks halogen bonds

Producing intermediates (molecules formed during a reaction)

called free radicals

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14
Q

what do free radicals do to alkanes

A

attack alkanes

lead to reactions:

initiation

propagation

termination

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15
Q

initiation

A

halogen broken down with UV light

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

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16
Q

propagation

A

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

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17
Q

termination

A

two radicals joing to end chain reaction

form one stable product

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18
Q

propagation extra

A

can result in multiple substitutions

chain reaction

conditions altered to favour terimnation step

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19
Q

alkenes

A

unsaturated hydrocarbons

carbon carbon double bond

area of high electron density - susceptible to attack from electrophiles

double bond has σ and π

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20
Q

bonds in alkenes

A
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21
Q

tests for alkenes

A

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

22
Q

Stereoisomers

A

same molecular formula

different spatial arrangement

23
Q

types of stereoisomers

A

E-isomer - high priority group diagonally across

Z-isomer - high priority group same side

24
Q

carbocations

A

primary carbocation

secondary carbocation

tertiary cation

reaction proceeds by most stable intermediate

in addition reactions multiple products can form

25
Q

Sigma bond σ

A

S orbitals overlap

can rotate

no change in molecule

alkanes form only sigma bonds

26
Q

Pie bond π

A

alkenes

2 pairs of electrons form between carbon atoms (one pair from each of the overlapping 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

27
Q

just hydrocarbons

A

Hydrogen groups on same side = CIS isomer

Hydrogen groups on opposite sides = TRANS isomer

28
Q

bonds can be broken hetrolytically

A

hetrolytic fission

29
Q

electrophile

A

species attracted to an area of high electron density

can accept a pair of e-

may or may not be charged

30
Q

electrophilic addition

A

happens to alkenes due to electron dense double bond

electrophiles attack double bond in alkenes

lead to formation of positive carbocation intermediate

31
Q

electrophilic addition of bromine

ethene

A
32
Q

asymetric alkene

A

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

33
Q

reaction of asymetric alkenes with H-BR

A
34
Q

reaction of an asymmetric alkene with H-Br

A
35
Q

overall reaction free radical substitution

A
36
Q

overall reaction free radical substitution

A
37
Q

single covalent bonds rotation

A

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.

38
Q

double carbon carbon bond

A

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.

39
Q

E -Z isomerism (also known as geometric isomerism)

A

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.

40
Q

atom priority E Z isomerisation

A

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

41
Q

2 geometric isomers of 1,2-dichloroethene

A
42
Q

single covalent bond formed by

A

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.

43
Q

double covalent bond formed by

A

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.

44
Q

ethene pi and sigma bonds

A
45
Q

reactivity of alkenes

A

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.

46
Q

hydrogenation of alkenes

A

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.

47
Q

electrophilic addition reactions of alkenes

  1. addition of hydrogen bromide to alkenes
A
  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.

48
Q

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

A

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.

49
Q

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

A

polymer

50
Q

Polymerisation of ethene

A

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.

51
Q

polymerisation of propene will produce the repeating unit:

A

2. Chemistry A-level (9 decks)

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