4.2 alcohols & haloalkanes

Question 1

state and explain the trend in boiling point of alcohols - 4 points

Answer 1

1. as chain length increases boiling point increases…
2. …because there are more electrons in longer chains…
3. …stronger dipole and induced dipole-dipole interactions…
4. …more energy needed to break the bonds.

Question 2

how does branching affect the boiling point of alcohols? state and explain - 4 points

Answer 2

1. more branching = lower boiling point
2. branches on chains mean there is less surface contact between molecules
3. so they are further away from each other
4. so the induced dipole-dipole interactions are reduced.

Question 3

what is the functional group of the alcohols and what is it called?

Answer 3

-OH it is the hydroxyl group

Question 4

why are alcohols polar? explain how this forms hydrogen bonds

Answer 4

they are polar because the hydroxyl group is electronegative
the electronegative O in the C-OH bonds draws electron density away from the H, making it delta+
this H attracts lone pairs from neighbouring Os in other molecules which creates hydrogen bonding.

Question 5

describe why alcohols are soluble in water

Answer 5

the alcohols form hydrogen bonds with the water molecules, allowing them to dissolve in the water

Question 6

compare the boiling points of alkanes and alcohols and explain this

Answer 6

alcohol bp > alkane bp
alcohols have higher boiling point due to hydrogen bonds in the -OH group
these bonds require more energy to break.

Question 7

why do alcohols have a relatively low volatility?

Answer 7

the strong hydrogen bonds prevent them from evaporating easily into a gas.

Question 8

what is a primary alcohol?

Answer 8

when the -OH group is attached to a C which is attached to ONE other C.

the C attached to the -OH group is attached to 2 hydrogens.

Question 9

what’s a secondary alcohol?

Answer 9

when the -OH group is attached to a C which is attached to TWO other Cs.

the C attached to the -OH group is attached to only ONE hydrogen.

Question 10

what is a tertiary alcohol?

Answer 10

when the -OH group is attached to a C which is attached to THREE other Cs.

the C attached to the -OH group has NO hydrogens attached to it.

Question 11

give the general word equation for the combustion of alcohols

Answer 11

alcohol + oxygen –> carbon dioxide + water

Question 12

name and give the symbol of the oxidising agent used int he oxidation of alcohols

Answer 12

acidified potassium dichromate - Cr₂O₇²⁻/H⁺

where K⁺ and SO₄²⁻ are spectator ions

Question 13

how does the acidified potassium dichromate tell us an oxidation has taken place?

Answer 13

it is an orange solution and turns green when it has been reduced/oxidised the alcohol

Question 14

which type of alcohol does not oxidise?

Answer 14

tertiary alcohols do not because they are very resistant to oxidation.

Question 15

what do secondary alcohols form when oxidised?

name the functional group and the stem of this molecule

Answer 15

ketone + water

ketone has functional group C=O and the stem is ‘one’ eg. methanONE, propanONE.

Question 16

name the two ways in which a primary alcohol can oxidise

Answer 16

by distillation and by reflux

Question 17

what is reflux? why is it used?

Answer 17

the constant boiling and condensing of a reaction mixture.

used to make sure the reaction goes to completion

Question 18

what is produced when a primary alcohol is oxidised under distillation?
name the functional group and stem of this product

Answer 18

aldehyde + water

aldehydes have functional group -CHO and stem ‘AL’ eg. methanAL, propanAL

Question 19

in an equation, what can an oxidising agent be abbreviated as?

Answer 19

[O]

Question 20

what is produced when a primary alcohol is oxidised under reflux?
name the functional group and stem of this product

Answer 20

carboxylic acid (water not produced)
FG: -COOH
stem: -oic acid, eg. menthanOIC ACID, propanOIC ACID

Question 21

carboxylic acid + alcohol –> ?

Answer 21

ester + water

Question 22

how can alcohols form haloalkanes? what catalyst is needed?

Answer 22

1. they react with compound containing halide ions eg. NaBr
2. the hydroxyl group is replaced by the halide and so it becomes a haloalkane
3. the reaction requires an acid catalyst, eg. H2SO4

Question 23

how can an alcohol become an alkene? what conditions are needed?

Answer 23

1. by eliminating the water in an alcohol (dehydrating it)
2. acid catalyst needed - concentrated sulphuric acid (
   H2SO4) or concentrated phosphoric acid (H3PO4)
3. the mixture must be HEATED

Question 24

name and describe two trends in the boiling point of haloalkanes

Answer 24

1. bp increases as C chain increases - more electrons - stronger induced dipole-dipole interactions - harder to break
2. bp decreases down - C-Cl bond has higher enthalpy - more energy needed to break
   C-I - lower bond enthalpy

Question 25

why can haloalkanes undergo nucleophilic substitution?

Answer 25

because the C-halogen bond is electronegative with the halogen being delta- and the C being delta+

Question 26

define nucleophile

Answer 26

an electron pair donor

Question 27

what’s it called when a haloalkane becomes an alcohol? state the conditions required for this reaction and give 3 steps of the reaction

Answer 27

hydrolysis - reacting with OH- ions 
MUST have a WARM AQUEOUS ALKALI
1. the delta+ C accepts the lone pair of electron from the nucleophile :OH-
2. this forms a C-OH bond 
3. the halogen is removed eg. :Br-

Question 28

how does the halogen on the alkane affect the rate of hydrolysis?

Answer 28

the higher polarity in the C-X bond, the stronger the bond.
eg. C-F is the most polar - strongest bond - HIGHER BOND ENTHALPY - so slower hydrolysis

C-I is least polar - weakest bond - low bond enthalpy

Question 29

how are radicals formed in the atmosphere?

Answer 29

CFCs in the atmosphere absorb UV light

Question 30

what do the radicals in the atmosphere contribute to?

write 4 equations of the CFC CF₂Cl₂ doing this

Answer 30

the depletion of the ozone layer
CF₂Cl₂ -> CF₂Cl• + Cl•
Cl• + O₃ -> ClO• + O₂ 
ClO• + O -> Cl• + O₂ 
O + O₃ -> 2O₂

Question 31

write 3 equations of how the radical •NO contributes to the ozone depletion

Answer 31

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

Question 32

how is the concentration of ozone in the atmosphere maintained? (use an equation)

Answer 32

O₃ ⇋ O + O₂

the rate of the forwards and backwards reaction is the same

Question 33

what is infrared spectroscopy used for?

Answer 33

identify a different compound

identifying functional groups

Question 34

how does IR spectroscopy identify compounds?

Answer 34

bonds absorb IR radiation and they vibrate

different bonds absorb different frequencies

Question 35

give 3 uses of infrared spectroscopy

Answer 35

• tracking green house gas levels
• monitor pollution from cars
• used in breathalysers - C-O is detected to find ethanol bc o-h bonds are in our breath anyway

Question 36

what is mass spectrometry used for?

Answer 36

molecular mass of compound

fragment patterns - make up of the compound

Question 37

on the mass spectra, where can you identify the molecular mass of the whole compound?

Answer 37

peak with the high m/z value is the overall mass

Question 38

what are the lower m/z peaks on the mass spectra called?

Answer 38

fragment ions