AQA AS Chemistry Unit 1 Revision Part 2 -Organic

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ORGANIC CHEMISTRY

What is an Organic Compound?

• An Organic compound contains carbon and hydrogen atoms covalently bonded.

Three Homologous Series(This is what you need to know for unit 1)

ALKANES (CnH2n+2):

hydrocarbons, mainly used as fuel

ALKENES (CnH2n):

hydrocarbons with double bonds, used to make polymers

HALOALKANES (CnH2n+1X):

contain halogen atoms, CFCs, which are being phased out

What is a Homologous Series?

Members of a homologous series all have:

the same general formulasimilar chemical propertiesphysical properties show a trendthe same functional group

Naming Alkanes

No of CarbonAtoms

Formula Name

Alkanes are named after the longest chain of carbon atoms in the molecule

Methane

Ethane

Propane

Butane

Pentane

Hexane

Remember - Monsters Eat Pretty Blondes!!!

Representing Formulae

• Molecular formula

• Actual numbers of atoms of each element present in a molecule e.g. C4H10

• Empirical formula

• Simplest ratio of atoms of each element present in a molecule e.g. C2H5

• Structural formula– Shows the structure of the molecule

H H H H

H—C—C—C—C—H

H H H H

The structural formula can be abbreviated to emphasise the carbon skeleton

CH3CH2CH2CH3

Displayed formula (shows every bond in the molecule)

Naming Alkanes

• Branches are named after the number of carbon atoms that they contain:

• 1 carbon = methyl

• 2 carbons = ethyl (very very rare for unit 1)

Naming Alkanes

• The position of the branch is indicated by a number

• The lowest possible number is used

H H H H|

H—C—H

2 - methylbutane

Naming Alkanes

H—C——C——C—H

H—C—H

| ||||

H—C—H

2,2-dimethylpropane

Naming Alkanes

• 2 of a functional group – di

• 3 of a functional group – tri

• 4 of a functional group – tetra

Cyclo alkanes• Rings of carbon atoms are known as cycloalkanes

• CnH2n

• Note – a cyclo alkane has the same molecular formula as the corresponding straight chain alkene.

• They are Functional group isomers – a type of structural isomer

• E.g.butene and cyclobutane are both C4H8

Naming haloalkanes

R-F: fluoroalkane R-Cl: chloroalkane R-Br: bromoalkane R-I: iodoalkane

Naming HaloAlkanes

The position of the halogen must be stated

Example 1-chloro propane 2-chloro

propaneH H

H—C—C—C—H

|Cl H H

H—C—C—C—H

Naming HaloAlkanes

If there are more than 1 halogen:-

they must be listed in alphabetic orderExample 2-bromo-1-chloropropane 2-chloro-1,1,1,

trifluoropropane

H—C—C—C—F

|Cl Br H

H—C—C—C—H

Naming Alkenes

• The position of the double bond is indicated by the number of the starting position

Example But-1-eneH H

H H H H

Rules for naming organic molecules

1. Count the longest chain. That gives the name of the chain

2. Look for branches and functional groups off the main chain

3. If 2 of the same – di, 3 of the same tri, 4 of the same tetra

4. Put a position number for each, remember start from the shortest end (lowest number.)

5. Where more than one group they must be in alphabetic order

STRUCTURAL ISOMERS

• These are compounds with the same molecular formula, but with different structural formulae

• e.g. Isomers of C4H10

H—C—C—C—H

|H H H H

H H H H

|H—C—H

There are three types of structural isomerism

Chain Isomerism Position Isomerism Functional Group Isomerism Remember - These are all forms of

structural isomerism - molecules have the same molecular formulae, but different structures

CHAIN ISOMERISM: the carbon chains are arranged differently.Example Butane and Methyl Propane

• Both Isomers of C4H10

H—C—C—C—H

|H H H H

H H H H

|H—C—H

POSITION ISOMERISM

the functional group is at a different position on the chain.

Example 1 chloro propane and 2 chloro propane

H—C—C—C—H

|Cl H H

H—C—C—C—H

FUNCTIONAL GROUP ISOMERISM

the isomers belong to different homologous series.

Example Butene and Cyclobutane

H—C—C

C—C—H

H H H H

THE OIL INDUSTRYTHE OIL INDUSTRY

Fractional DistillationFractional Distillation

• Crude oil consists of a large number of different hydrocarbons.

• Most of these hydrocarbons are alkanes.

• The alkanes are separated according to their size by fractional distillation.

• Click here to see fractional distillation tower

Fractional Distillation

• Crude oil is heated and vapour passed into the bottom of a tower

• Top of tower cooler than bottom – I.e. temperature gradient

• This separates mixture into fractions depending on boiling points of hydrocarbons present

• Only those with low boiling points reach top• Others condense in trays at different levels up the

tower and are drawn off here

Questions• What is passed into the bottom of the tower

• Which hydrocarbons will be piped off at the bottom of the tower

• Why?

CrackingCracking

• Cracking converts large alkanes into smaller ones which are more useful for making plastics or motor fuel.

Types of CrackingTypes of Cracking• THERMAL CRACKING• 700 – 1200 K , 70 atmos• Makes alkenes for the plastics

industry.

Types of CrackingTypes of Cracking• CATALYTIC CRACKING• 720K; 1 atmos; • zeolite catalyst consisting of Silicon

dioxide and Aluminium oxide• Products are mainly Branched and

cyclic products, • Products mainly for motor fuel

Example and Exercises• Cracking of Decane• Decane → Octane + Ethene• C10H22 → C8H18 + C2H4

• Exercises

1. Write an equation for the thermal cracking of one molecule of C10H22 to give one molecule of propene and one molecule of an alkane only.

2. Write an equation to show the cracking of C14H30 to produce two molecules of butene plus one other molecule.

3. Write an equation to show the cracking of C14H30 to produce hexane and ethene

4. Write an equation to show the cracking of C14H30 to produce one molecule of propene and one molecule of propane and one other molecule

5. Write an equation to illustrate the thermal cracking of one molecule of tetradecane, C14H30in which the products are ethene and propene in the ratio of 2:1, and one other product

Combustion of AlkanesComplete

Combustion is reaction with oxygen. Complete combustion of a hydrocarbon

produces water plus carbon dioxide. Example: ETHANE + OXYGEN → CARBON DIOXIDE

+ WATER

• C2H6 + 3½O2 → 2CO2 + 3H2O

Combustion of AlkanesLimited oxygen

When the supply of oxygen is limited, incomplete combustion occurs, and instead of CO2, carbon and carbon monoxide may be formed.

Formation of carbon monoxide (CO: toxic)

• C2H6 + 2½O2 → 2CO + 3H2O

Formation of carbon (soot)

• C2H6 + 1½O2 → 2C + 3H2O

POLLUTION PROBLEMS

Burning fossil fuels causes these pollutants:

Types of PollutionCAUSE POLLUTANT PROBLEM

S in fossil fuels reacts with air when burnt

SO2 Produces Acid Rain

High temperature in car engine N2 and O2 in air react

NXOY: : NO, NO2, N2O

Causes acid rain and smog. Makes asthma, bronchitis worse

Lack of oxygen in car engine Heaters not serviced

CO C Unburned hydrocarbon

CO - Toxic C - Unsightly Causes cancer, makes asthma, bronchitis worse

Exercise: write equations for the reactions below.

The formation of the following oxides of nitrogen from N2 and O2 in the car engine.

b. N2O

c. NO2

d. N2O4

Ways to reduce pollution

• Fit catalytic converters in cars• Use filters in power station chimneys to remove

Sulphur dioxide ( CaO)

Power Stations Many power stations burn fossil fuels. Sulphur impurities in fossil fuels react to form SO2,

which causes acid rain. Equation: S + O2 → SO2

SO2 + H2O → H2SO3

H2SO3 + ½O 2 → H2SO4 Sulphur dioxide can be removed from emissions by

reaction with calcium oxide. Equation: SO2 + CaO + ½O 2 → CaSO4 The calcium sulphate is used as plaster. This is a neutralisation reaction The process is called flue gas desulfurisation

Catalytic Converters

Pollution from cars can be reduced by fitting a catalytic converter.

This causes the pollutants to react amongst themselves.

Less harmful substances are formed. Platinum or Rhodium Catalyst Watch this video clip

Summary of reactions in catalytic converter

In Out

CO CO2

NOx N2

Unburned CO2 + H2O

Hydrocarbons

Catalyst ConverterExamples of reactions

• 2NO + 2CO → N2 + 2CO2

• 2NO2 + 4CO → N2 + 4CO2

• N2O + CO → N2 + CO2

• C5H12 +16NO → 8N2 + 5CO2 + 6H2O

Exercise

Write equations to show the reaction of these alkanes with NO:

DECANE (C10H22)

CYCLOHEPTANE (C7H14)

BUTANE (C4H10)

OCTANE (C8H18)

Global Warming and the Greenhouse effect

Visible radiation (sunlight) readily penetrates atmosphere and warms the earth

Invisible infrared radiation is emitted by the earth and cools it down.

Carbon dioxide traps infrared preventing it escaping – earth’s atmosphere warms up (good in small amounts, bad if too much)

Level of Carbon dioxide has been rising since industrial revolution, causing global warming

Carbon dioxide is called a greenhouse gas Other greenhouse gases are water vapour and methane, the

concentration of these in atmosphere tends to stay roughly the same

Carbon Neutral Activities

These are activities that produce no overall carbon dioxide emissions into the atmosphere

AQA AS Chemistry Unit 1 Revision Part 2 -Organic

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