4.2 Alcohols, Haloalkanes And Analysis

Question 1

Describe the polarity of alcohols

Answer 1

They are generally polar due to the electronegative hydroxyl group which pulls the electrons in the C-OH bond away from the carbon atom

Question 2

Describe the trend in solubility of alcohols

Answer 2

In larger alcohols, most of the molecule is a non polar carbon chain, so there’s less attraction for the H2O molecules.

So increase in alcohol size, decreases in water solubility

Question 3

Compare the volatility and boiling point of alcohols to alkanes

Answer 3

Alkanes have a much lower boiling point than an alcohol with the same number of carbons

• In alkanes the intermolecular forces are only weak induced dipoles (London Forces)
• In alcohols, the OH group can form hydrogen bonds, which are much stronger
• These hydrogen bonds require much more energy to be broken
• So boiling points are higher and they are less volatile.

Question 4

How can you tell whether an alcohol is primary, secondary or tertiary?

Answer 4

Primary- OH attached to a C that is attached to 1 other Carbon

Secondary- OH attached to a C that is attached to 2 Carbons

Tertiary- OH attached to a C that is attached to 3 Carbons

Question 5

Write an equation for the combustion of ethanol

Answer 5

C2H5OH + 3O2 —> 2CO2 + 3H2O

Question 6

What oxidising agents can be used to oxidise alcohols

What colour is at first and after it reacts?

Answer 6

Acidified dichromate(vi) mildly oxidies alcohols
Orange dichromate(vi) reduced to Green chromium(iii)

Cr2O7^(2–) /H+
eg.
K2Cr2O7 or H2SO4

Question 7

What happens when you distill a primary alcohol?
and oxidise again under reflux?

(With oxidising agent)

Answer 7

Makes aldehyde and H2O

Makes carboxylic acid

Question 8

What happens when you reflux a secondary alcohol?

With oxidising agent

Answer 8

Makes Ketones and H2O

Question 9

What is an oxidising agent represented as?

Answer 9

[O]

Question 10

What happens when you oxidise a tertiary alcohol?

Answer 10

They can’t. As No hydrogen atoms on the carbon atom that holds the alcohol group

Question 11

What is it meant by dehydration of an alcohol? (elimination reaction)

(Reactants, products, conditions?)

Answer 11

• H2O can be eliminated from alcohols to give ALKENES

- Heat under reflux with a strong acid catalyst eg. concentrated H2SO4/H3PO4

Question 12

What are some things to be careful of for dehydration of alcohol questions?

Answer 12

• Often 2 possible alkene products from one elimination reaction depending on which side of the hydroxyl group the hydrogen is eliminated from
• watch out if alkene products can form E/Z isomers (if they can mix of both isomers form)

Question 13

What is meant by substitution reaction of alcohol with halide ion?

(Alcohol to haloalkane)

Give conditons.

Answer 13

-OH group replaced by Cl-/Br-/I-
•ROH + HX –> RX + H2O (X halogen)

• H2SO4 is added along with the NaX and the mixture is WARMED (20.c)
• This doesn’t work for iodine as it’s oxidised by H2SO4 to give I2 so the yield is poor so H3PO4 instead

Question 14

What is meant by hydrolysis of haloalkane in a substitution reaction by aqueous alkali?

(Haloalkane to alcohol)

Give conditons.

Answer 14

This is where a nucleophile (an electron pair donor)
usually OH- reacts with a halogenoalkane due to its
polar C–X bond (x = halogen).

The OH- donates its electrons to the polar C, this than
causes the C–X bond to break and X– is formed and alcohol.

NaOH or KOH used
Done under reflux conditions

Question 15

How could you compare experimentally the rates of hydrolysis of different carbon-halogen bonds?

Why cant flouroalkanes be used?

Answer 15

Haloalkanes react with AgNO3 and water
•Halide ion leaves during the reaction
•This reacts with Ag+ to give a precipitate
an use how long the precipitate takes to appear to determine how fast reactions of different haloalkanes are
(EXPERIMENT)
•Use this to determine bond strengths of different C-X bonds
•Stronger bonds will lead to a slower reaction so precipitate will take longer to appear
•Fluoroalkanes can’t be used because FNO3 is soluble, not a precipitate

Question 16

What are the main steps of the experiment to determine rates of hydrolysis of different carbon-halogen bonds?

Answer 16

○Add 1cm3 of each haloalkane to different test tubes in a water bath at 50oc
○Put test tubes with ethanol, water and aqueous silver nitrate in the water bath
○Allow them to come to the same temperature
○Add 1cm3 of ethanol, water and aqueous silver nitrate to each haloalkane
○See how long it takes for precipitate to appear

Question 17

What is a nucleophile?

Answer 17

Electron pair donor

Question 18

Describe the mechanism for nucleophilic substitution in the hydrolysis of primary haloalkanes with aqueous alkali

Answer 18

Nucleophile (mark on lone pair and negative charge,
:-OH )
Curly arrow to C δ+ attached to halogen in the hydrocarbon
Curly arrow from C-X bond to X (mark x with δ-)

ARROW–>

Haloalkane + :X-

Question 19

What does branching do to volatility and bp?

Answer 19

Branching generally decreases boiling point, and so increases volatility

Question 20

Explain the trend in the rates of hydrolysis of primary haloalkanes in terms of the bond enthalpies of carbon–halogen bonds (C–F, C–Cl, C–Br and C–I)

Answer 20

• Going down halogens atoms increase in size
• Bonding electrons are further from nucleus so more shielded
• This gives lower C-X bond enthalpies
• Easier to break the bond so reaction happen faster

Question 21

What are CFCs?

Answer 21

Chlorofluorocarbons, haloalkanes.

Question 22

What are properties of CFC’s?

Answer 22

Stable, volatile, nonflammable, non toxic

Question 23

What were the uses of CFC’s?

Answer 23

Fridges, aerosol cans, dry cleaning and air-con

Question 24

What could you say ozone in the upper atmosphere acts like?

Answer 24

Chemical sunscreen, it absorbs lots of UV radiation which can cause sunburn and skin cancer

Question 25

How is ozone formed?

Give equation

Answer 25

Oxygen molecule is broken down into 2 free radicals by UV radiation. The free radicals attack other Oxygen molecules forming ozone.

O2—UV–> O. + O.
O2 + O. –> O3

Question 26

Why are holes in Ozone layer bad?

Answer 26

Allows more harmful UV radiation to reach earth

Question 27

How are holes in the Ozone layer formed?

Answer 27

• CFC’s in the upper atmosphere absorb UV radiation and split to form chlorine free radicals.
• The radicals catalyse the destruction of the ozone layer, they destroy ozone molecules and regenerate to destroy more.

Question 28

Give equation for production of halogen radicals
And the breakdown of ozone
what is the Cl•

Answer 28

CF2Cl2(g) —UV—> •CF2Cl(g) +Cl•

Propagation: Cl•(g) + O3(g) —> O2(g) + ClO•(g)
Propagation: ClO•(g) + O(g) —>O2(g) + Cl•(g)

Overall: 03(g) + O(g) —> 2O2 (g)

The Cl• is the catalyst

Question 29

Give the equations for when nitrogen oxides break ozone down too

Answer 29

Propagation: ·•NO + O3 —> •NO2 + O2
Propagation: •NO2 + O —> •NO + O2
Overall: O3 + O 2O2

Question 30

Where are nitrogen oxides produced?

Answer 30

Car, aircraft engines, thunderstorms

Question 31

What are the alternatives to CFC’s and whats the problem with them?

Answer 31

HCFC’s-still damage ozone layer, greenhouse gas

HFC’s-greenhouse gas

Question 32

What do they use nowadays for aerosol cans and frdges?

Answer 32

Aerosol cans- Pump spray system or nitrogen propellant

Frdge- ammonia as coolant gas

CO2 to make foamed polymers

Question 33

The techniques and procedures for:

Use of Quickfit apparatus including for distillation and heating under reflux

Answer 33

For reflux – vertical condenser attached to flask, the water goes into the bottom of the condenser, often use water bath (flask in beaker of water) or heating element (electric heater) to heat flask rather than direct
heating with Bunsen burner as chemicals may be flammable.

For distillation – set up condenser at an angle with the higher end at flask and sloping away, the water is attached to lower end, the thermometer should be level with entrance to condenser

Question 34

Describe Preparation and purification of an organic liquid

Answer 34

Use of a separating funnel to remove an organic
layer from an aqueous layer
The aqueous and organic layer don’t mix, you can
drain off bottom layer to separate the two.

Drying with an anhydrous salt (e.g. MgSO4, CaCl2)
Add anhydrous salt which will absorb any water,
filter the mixture to remove the now hydrated salt
and leave product.

Redistill - distillation as before to purify product.

Question 35

What happens when chemical absorb IR

Answer 35

their covalent bonds vibrate more and absorb energy

Question 36

What is the use of IR spectra?

Answer 36

Used in monitoring air pollution
•Pollutants like CO and NO have specific absorptions that allow their concentrations to be monitored
•This can be used to study the effect of traffic on air quality
•Also used in breathalyzers

Question 37

What are some common fragment ion peaks?

Answer 37

15 = CH3+, 
29 = C2H5+, 
17 = OH+,

Question 38

Explain how you would carry out separation to remove water soluble impurities from a product?

Answer 38

• Mixture poured in separating funnel
• Water is added
• Funnel is shaken and allowed to settle
• Organic layers is less dense so floats on top
• Water soluble impurities should dissolve in lower aqueous layer
• Open stopper to let aqueous layer drain off

Question 39

Whats the problem with using separation? How do you combat this issue?

Answer 39

Organic layer will contain trace amounts of water, so must be dried

-Add anhydrous salt (MgSO4, CaCl2), this is a drying agent and binds to water becoming hydrated

-When you first add salt to organic layer it will be lumpy
so add more salt

• You know when All water is removed as when you swirl the mixture it looks like snow globe
• Filter mixture to remove solid drying agent