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OCR A a-level chemistry > Module 4 - [ch14,15] (alcohols, haloalkanes) > Flashcards

Module 4 - [ch14,15] (alcohols, haloalkanes) Flashcards

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1
Q

why is methanol useful

A

efficient combustion - used as a high performance fuel
starting material in many industrial syntheses
(e.g to produce paint, adhesives ect.)

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2
Q

physical properties of alcohols in comparison to alkanes

A

higher melting points
more soluble in water

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3
Q

why are alcohols more soluble than alkanes in water

A

can form hydrogen bonds with water molecules

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4
Q

why do alcohols have a higher boiling points than alkanes

A

alcohol has hydrogen bonds
stronger than london forces in alkanes
require more energy to overcome
less volatile than alkanes

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5
Q

describe hydrogen bonding in alcohols

A

alcohols have a polar O-H bond due o difference in electronegativity of the oxygen and hydrogen atoms
therefore polar
-OH groups form strong hydrogen bonds

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6
Q

how does solubility of alcohols differ to that of alkanes as chain length increases

A

as hydrocarbon chain length increases, the influence of the -OH decreases
solubility of longer-chain alcohols becomes to similar to hydrocarbons

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7
Q

how are alcohols classified

A

primary, secondary and tertiary
based on number of hydrogen atoms and alkyl groups attached to the carbon atom with the -OH group

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8
Q

what are the two primary alcohols

A

methanol, ethanol

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9
Q

what do you call an alcohol where the carbon with the -OH group is bonded to no hydrogen atoms and 3 alkyl groups

A

tertiary

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10
Q

combustion of alcohol

A

alcohol + oxygen -> carbon dioxide + water

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11
Q

oxidation of alcohols

A

alcohol + oxidising agent -> carboxylic acid

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12
Q

which alcohols can be oxidised

A

primary and secondary

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13
Q

the example of oxidation you need to know

A

potassium dichromate(VI)
K2Cr2O7
acidified with dilute sulfuric acid
H2SO4
oxidised - orange -> green
orange ( dichromate(VI) ions)
green (chromium(III) ions)

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14
Q

what are primary alcohols oxidised to

A

aldehydes or carboxylic acids

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15
Q

how do you prepare an aldehyde

A

gentle heating of primary alcohol with acidified potassium dichromate

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16
Q

why is aldehyde is distilled out of the reaction mixture as it forms

A

to prevent further reaction with the oxidising agent
aldehydes can be oxidised to carboxylic acids

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17
Q

how do you prepare carboxylic acids

A

heat primary alcohol strongly under reflux with an excess of acidified potassium dichromate(VI)

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18
Q

why is an excess of acidified potassium dichromate (VI) used in preparation of a carboxylic acid

A

ensures all of the alcohol is oxidised

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19
Q

why do you heat under reflux when preparing a carboxylic acid from a primary alcohol

A

ensures any aldehyde formed initially in the reaction also undergoes oxidation to become a carboxylic acid

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20
Q

oxidation of secondary alcohols

A

ketones

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21
Q

what is done to ensure the oxidation of secondary alcohols goes to completion

A

the secondary alcohol is heated under reflux with the oxidising mixture
dichromate, orange-.green

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22
Q

what is done to ensure the oxidation of secondary alcohols goes to completion

A

the secondary alcohol is heated under reflux with the oxidising mixture
dichromate, orange-.green

23
Q

oxidation of tertiary alcohols

A

do not undergo oxidation reactions
acidified dichromate remains orange

24
Q

describe dehydration of alcohols

A

alcohol heated under reflux
in the presence of an acid catalyst (H2SO4, H3PO4)
to form an alkene + water

25
Q

what type of reaction is the dehydration of an alcohol

A

elimination

26
Q

alcohol + hydrogen halide

A

haloalkanes

27
Q

how do you prepare a haloalkane

A

alcohol heated under reflux
with sulfuric acid and a sodium halide
substitution reaction

28
Q

are haloalkanes reactive and why

A

yes
carbon - halogen bond
halogen more electronegative than carbon
electron pair closer to halogen atom
polar bond
carbon has a delta positive charge
acts as a electrophile and attracts a nucleophile

29
Q

nucleophilic substitution

A

when a haloalkane reacts with a nucleophile
nucleophile replaces the halogen in a substitution reaction

30
Q

three most common nucleophiles

A

hydroxide ions
water molecules
ammonia molecules

31
Q

nucleophile

A

lone electron pair donor

32
Q

what is hydrolysis

A

chemical reaction involving water or an aqueous solution of a hydroxide that causes the breaking of a bond in a molecule

33
Q

hydrolysis of a haloalkane

A

halogen atom is replaced by an -OH group
nucleophilic substitution

34
Q

describe the mechanism for hydrolysis of a haloalkane

A

nucleophile OH~ approaches the carbon attached to the halogen from the other side
direction of attack minimises repulsion between negative charges
bond formed between carbon atom and oxygen of hydroxide ion
C - halogen bond broken by heterolytic fission
alcohol and halide ion formed

35
Q

how can haloalkanes be converted to alcohols (not hydrolysis)

A

aqueous sodium hydroxide (NaOH)
slow reaction at room temp
mixture heated under reflux to obtain good yield

36
Q

what does the rate of hydrolysis depend on

A

strength of the carbon-halogen bond

37
Q

how do the carbon-halogen bonds vary

A

C-F is the strongest
largest difference in electronegativity
unreactive as much more energy required to break this bond rather than C-I

38
Q

how would you measure the rate of hydrolysis

A

haloalkane + water
carry out these reactions in presence of aqueous silver nitrate
Ag+ ions react with the X~ ions as they are formed to form precipitate AgX

AgCl - white
AgBr - cream
AgI - yellow

39
Q

why is ethanol solvent used rather than water in the hydrolysis of primary haloalkanes

A

ethanol allows water and the haloalkane to mix and produce a single solution rather than two layers
haloalkanes are insoluble in water

40
Q

the rate of hydrolysis increase as…

A

carbon-halogen strength decreases

41
Q

what are organohalogen compounds

A

molecules that contain at least one halogen atom joined to the carbon chain

42
Q

uses of organohalogen compounds

A

pesticides
making polymers
flame retardants
dry cleaning solvents

43
Q

what are CFCs

A

chlorine-fluoro-carbons
haloalkanes containing only Cl,F,C

44
Q

what are the issues regarding CFCs

A

catalyse the breakdown of the ozone layer
via free radical substitution

45
Q

what is the main function of the ozone layer

A

provides protection from harmful UV radiation

46
Q

does the ozone play a protective role everywhere

A

contributes to photochemical smog in the trotosphere

47
Q

what does ozone decompose to
write an equation for this

A

oxygen
2O3 -> 3O2

48
Q

where is the ozone layer

A

outer edge of the stratosphere
height from 10-40km above earth’s surface

49
Q

how is ozone formed

A

O2 broken into oxygen radicals
initially high energy UV breaks oxygen molecules
O2 + O -> O3
steady state set up involving formation of ozone at same rate of breaking down of the ozone

use of CFCs and other human activity has upset this delicate equilibrium

50
Q

impact of CFCs on the earth’s atmosphere

A

CFCs remain stable until they reach the stratosphere
begin to break down to form chlorine radicals
which catalyse the breakdown of the ozone layer

51
Q

what is photodissociation

A

when radiation initiates the breakdown

52
Q

free radical substitution reactions to show how cl catalyses the breakdown of O3

A

Cl2 -> 2Cl .
Cl . + O3 -> ClO. + O2
ClO. + O3 - > 2O2 + Cl .

overall equation
2O3 - > 3O2

53
Q

free radical substitution equations to show how nitrogen monoxide can decompose the ozone

A

.NO + O3 -> NO2. + O2
NO2. + O -> NO. + O2
O3 + O -> 2O2

54
Q

how are nitrogen oxide radicals formed

A

naturally during lightning strikes
as a result of aircraft flying in the stratosphere

OCR A a-level chemistry (11 decks)

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