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OCR A a-level chemistry > Module 4 - [ch12,13] (alkanes alkenes) > Flashcards

Module 4 - [ch12,13] (alkanes alkenes) Flashcards

alkanes alkenes

1
Q

what are alkanes

A

crude oil
among the most stable organic compounds
unreactive
used as fuels
saturated hydrocarbons

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2
Q

what does the bonding look like in alkanes

A

saturated hydrocarbons
only single covalent bonds
each carbon makes 4 bonds

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3
Q

what is the shape of alkanes

A

each carbon atom is surrounded by 4 bonded pairs
tetrahedral
109.5

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4
Q

What is meant by the term fractional distillation? [1]

A

separation by (differences in) boiling point

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5
Q

why does boiling point of alkanes increase with chain length

A

as chain length increases, molecules have larger surface area
more surface contact possible between molecules
london forces between molecules are greater so more energy is required to overcome these forces

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6
Q

what is the effect of branching on boiling point of alkanes

A

branched isomers have lower boiling points

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7
Q

what are the reasons for the lack of reactivity of alkanes

A

C - C C - H
sigma bonds are strong
C - C are non-polar
barely any difference in electronegativity of C and H so can also be described as non-polar

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8
Q

combustion of alkanes

A

alkane + oxygen -> carbon dioxide + water

gives out heat

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9
Q

incomplete combustion of alkanes

A

limited supply of oxygen
toxic gases such a carbon monoxide (CO)
soot

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10
Q

when do alkanes react with halogens

A

in the presence of sunlight
UV radiation provides initial energy for reaction to take place
substitution reaction
methane + bromine -> bromomethane

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11
Q

mechanism from bromination of alkanes

A

initiation
Br - Br -> Br. + Br.
homolytic fission

propagation
CH4 + Br. -> CH3. + HBr
CH3. + Br2 -> CH3Br + Br.

termination
Br. + Br. -> Br2
CH3 + CH3 -> C2H6
CH3. + Br. -> CH3Br

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12
Q

what are the limitations to radical substitution

A

further substitution
substitution at different positions on the carbon chain

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13
Q

what are alkenes

A

unsaturated hydrocarbons
contain a C=C double bond

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14
Q

how is a pi bond formed

A

overlap of adjacent p-orbitals below and above the Carbon atoms
can only be made after a sigma bond is formed

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15
Q

describe the shape around a double bond

A

three regions of electron density around each carbon atom
trigonal planar
120

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16
Q

what are stereoisomers

what are the two types

A

they have the same structural formula but different arrangement of the atoms in space

EZ isomerism (only in compounds with C=C)
optical isomerism

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17
Q

what are the conditions for EZ isomerism

A

. C=C double bond
. different groups attached to each carbon atom of the double bond

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18
Q

what is cis-trans isomerism

A

a special case of EZ isomerism
where one of the attached groups on each carbon atom of the double bond must be the same

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19
Q

how do you assign priority

A

examine the atom directly attached to the carbon atoms of the double atom
greater atomic number = higher priority
if there is no difference, continue down chains till point of difference and compare those atoms

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20
Q

cahn-ingold

A

if highest priority groups are on the same side
(Z)
on opposite sides
(E)

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21
Q

why are alkenes more reactive than alkanes

A

the pi-bond
it is outside of the double bond, therefore more exposed
a pi bond readily breaks and alkenes undergo addition reactions easily

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22
Q

hydrogenation of alkenes

A

alkene + H2 -> alkane

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23
Q

halogenation of alkenes

A

alkene + halogen -> something

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24
Q

how do you test for an alkene

A

bromine water tests for unsaturation (the double bond)
orange -> colourless

25
Q

alkene + hydrogen halide

A

haloalkanes

26
Q

hydration of alkenes

A

alkene + water -> alcohol

27
Q

what does the double bond in an alkene represent

A

a region of high electron density
because of the presence of the pi-electrons
this high electron density attracts electrophiles

28
Q

what is the mechanism for the addition of a hydrogen halide

A

Hydrogen halide is polar
has dipoles
positive dipole attracted to double bond (high electron density)
double bond break open and H-Br bond also broken by heterolytic fission, electron pair goes to bromine atom
bromide ion and carbocation formed
carbocation has a positively charged carbon atom which attracts the ion
product formed!

29
Q

what makes the mechanism of the electrophile addition of Br2 different to the addition of HBr

A

Br2 is non-polar
when it approaches the double bond
pi-electrons interact with the electrons in the Br2
induced dipole

30
Q

What is Markovnikoff’s rule

A

when a hydrogen halide reacts with an unsymmetrical alkene the hydrogen attaches itself to the carbon with the greater number of hydrogen atoms and least carbon atoms

31
Q

carbocation stability

A

number of ‘alkyl’ groups attached to the positively charged carbon, more alkyl groups -> more stable
(how many carbons are attached to it)
primary carbocations are the least stable, tertiary the most

32
Q

why are tertiary carbocations the most stable

A

more alkyl groups
the charge is more spread out
making the ion more stable

33
Q

what are polymers

A

extremely large molecules made up from repeating units called monomers

34
Q

why am i waffling about polymerisation in this topic

A

alkene molecules undergo addition polymerisation
open the double bond
square brackets
slap an ‘n’ at the bottom right corner
!polymer!

35
Q

uses of polyethene

A

most commonly used polymers
supermarket bags
toys
shampoo bottles

36
Q

uses of polychloroethene

A

PVC
pipes, flooring, bottles, insulation

37
Q

polytetrafluoroethene (teflon)

A

coating non-stick pans
permeable membrane for clothing

38
Q

polystyrene (polyphenylethene)

A

packaging material
thermally insulating

39
Q

environmental concerns regarding polymers

A

disposal -> non-biodegradable

recycling PVC
hazardous due to high chlorine content
when burnt releases hydrogen chloride - a corrosive gas

feedstock recycling - reclaiming monomers from waste polymers

40
Q

bioplastics

A

produced from plant starch, cellulose

41
Q

biodegradable polymers

A

broken down by microorganisms into water, cO2 and biological compounds
compostable polymers

42
Q

photodegradable polymers

A

weakened by absorbing light

43
Q

what are the conditions for halogenation of alkenes

A

rapid addition reaction
room temp
addition reaction

44
Q

what are the conditions for hydrogenation of alkenes

A

nickel catalyst 150 degrees, 423K
addition reaction
double bond breaks, addition of hydrogen

45
Q

what are the conditions for hydration of alkenes

A

alkenes react with steam (H2O)
presences of phosphoric acid catalyst (H3PO4)
addition reaction

46
Q

electrophilic addition
alkene + hydrogen halide
what are the conditions

A

room temp
addition reaction
(bubbles through if the alkene is liquid)

47
Q

are alkane bonds polar

A

no
carbon and hydrogen have similar electronegativites

48
Q

describe the sigma bond in an alkane

A

covalent bond
direct overlap of s-orbitals

49
Q

what type of intermolecular force do alkanes have and why

A

london forces
because bonds are non-polar

50
Q

are alkanes soluble in water
why?

A

insoluble
hydrogen bonds in water are stronger than the london forces in the alkane
no polar bonds, so no attraction between water and alkane molecules

51
Q

what reactions will alkanes undergo

A

combustion
reaction with halogens

52
Q

what is the colour of the flame on a bunsen burner during complete combustion

A

blue

53
Q

how are halogenoalkanes formed from alkanes

A

radical substitution

54
Q

what intermolecular forces do alkenes have and why

A

london forces
non-polar bonds

55
Q

what will major products be formed from

A

the most stable carbocation
tertiary

56
Q

what are 5 ways in which plastic is disposed of

A

landfill
combustion
organic feedstock
recycling
reuse

57
Q

what is organic feedstock

A

plastics are separated and broken down into small organic molecules

58
Q

Outline two ways that waste polymers are processed usefully, rather than just dumped
in landfill sites [2]

A

combustion for energy generation
feedstock for chemicals, or plastics

59
Q

why do branched alkanes have lower boiling points

A

have fewer surface points of contact between molecules
weaker london forces
branches mean the molecules can’t get close together
decreasing intermolecular forces

OCR A a-level chemistry (11 decks)

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