Organic Compounds - Alkanes,enes,acids

Question 1

Alkanes

Answer 1

A homologous series of hydrocarbon compounds with only single carbon bonds, there are no C=C bonds present. General Formula: CnH2n+2

Question 2

Saturated hydrocarbons?

Answer 2

A compound that contains only carbon carbon signle bonds with no carbon carbon double or triple bonds.

Question 3

Reactivity of alkanes?

Answer 3

Alkanes are generally unreactive compounds but they do undergo combustion reactions, can be cracked into smaller molecules and react with halogens in the presence of light.

Question 4

Substitution Reaction of Alkanes with Halogens

Answer 4

In a substitution reaction, one atom is swapped with another atom Alkanes undergo a substitution reaction with halogens in the presence of ultraviolet radiation.

In the presence of ultraviolet (UV) radiation, methane reacts with bromine in a substitution reaction

Methane + Bromine → Bromomethane + Hydrogen Bromide

CH4 + Br2 → CH3Br + HBr

Question 5

Describe the bonding in alkanes

Answer 5

• Saturated (only contain single covalent bonds).
• Each carbon atom forms four covalent bonds.
• Contain C-C and C-H covalent bonds.

Question 6

What is required for alkanes to react with chlorine?

Answer 6

UV radiation

Question 7

Write the word and chemical equations for the reaction between methane and chlorine.

Answer 7

Methane + chlorine → methyl chloride + hydrogen chloride CH4 + Cl2 → CH3Cl + HCl

Question 8

Describe how alkenes can be manufactured

Answer 8

• Cracking allows large hydrocarbon molecules to be broken down into smaller, more useful hydrocarbon molecules
• Fractions containing large hydrocarbon molecules are heated at 600 – 700°C to vaporise them
• Vapours will then pass over a hot catalyst of silica or alumina
• This process breaks covalent bonds in the molecules, causing thermal decomposition reactions
• As a result, cracking produces smaller alkanes and alkenes. The molecules are broken up in a random way which produce a mixture of alkanes and alkenes
• Hydrogen and a higher proportion of alkenes are formed at temperatures of above 700ºC and higher pressure

Question 9

catalytic cracking?

Answer 9

A process called catalytic cracking is used to convert longer-chain molecules into short-chain and more useful hydrocarbons

Alkenes and hydrogen are produced from the cracking of alkanes

Kerosene and diesel oil are often cracked to produce petrol, other alkenes and hydrogen

Question 10

Distinguishing between alkanes and alkenes

Answer 10

• Alkanes and alkenes have different molecular structures
• All alkanes are saturated and alkenes are unsaturated
• The presence of the C=C double bond allows alkenes to react in ways that alkanes cannot
• This allows us to tell alkenes apart from alkanes using a simple chemical test:

Question 11

What’s the difference between saturated and unsaturated compounds?

Answer 11

Saturated - only contain single covalent bonds.

Unsaturated - contain at least one double bond.

Question 12

How can you distinguish between saturated and unsaturated compounds?

Answer 12

Add bromine water. Bromine water is decolourised in unsaturated compounds (alkenes) whereas it remains orange in saturated compounds (alkanes).

Question 13

Why do alkenes decolourise bromine water?

Answer 13

Alkenes are unsaturated. The double bond allows alkenes to react with bromine to form a bromoalkane.

Question 14

Alkenes?

Answer 14

A homologous series of hydrocarbon compounds with carbon-carbon double bonds (C = C)

General formula:

CnH2n

Question 15

Catalytic cracking explanation in Alkenes (PICTURE):

Answer 15

The 10 carbon molecule decane is catalytically cracked to produce octane for petrol and ethene for ethanol

Question 16

Addition Reaction?

Answer 16

A reaction in whch a simple molecule adds across the carbon carbon double bond of an alkene.

Bromine water is an orange coloured solution of bromine

• When bromine water is shaken with an Alkane, it will remain as an orange solution as alkanes do not have double carbon bonds (C=C) so the bromine remains in solution
• But when bromine water is shaken with an alkene, the alkene will decolourise the bromine water and turn colourless as alkenes do have double carbon bonds (C=C)
• The bromine atoms add across the C=C double bond hence the solution no longer contains the orange coloured bromine
• This reaction between alkenes and bromine is called an addition reaction

Question 17

What are the different products formed when an alkene undergoes addition reactions with bromine, hydrogen and steam?

Answer 17

Addition with bromine: Dibromoalkane.
Addition with hydrogen: Alkane.
Addition with steam: Alcohol

Question 18

In terms of alkenes, what is an addition reaction?

Answer 18

The addition of hydrogen, a halogen or steam across the C=C double bond. The product doesn’t contain a C=C double bond.

Question 19

Write the word and balanced symbol equations for the reaction between ethene and bromine

Answer 19

Ethene + Bromine → 1,2-dibromoethane

C2H4 + Br2 → C2H4Br2

Question 20

Write the word and balanced symbol equations for the reaction between propene and hydrogen

Answer 20

Propene + hydrogen → Propane C3H6 + H2 → C3H8

Question 21

Write the word and balanced symbol equations for the reaction between ethene and steam

Answer 21

Ethene + steam → ethanol C2H4 + H2O → C2H5OH

Question 22

Addition Polymerisation

Answer 22

• Addition polymers are formed by the joining up of many small molecules called monomers
• Addition polymerisation only occurs in monomers that contain C=C bonds
• One of the bonds in each double bond breaks and forms a bond with the adjacent monomer
• There are many types of polymers that are synthesized from alkene monomers
• A common example is poly-ethene (polythene) which is the addition of many ethene monomers

Question 23

Hydrogenation reactions?

Answer 23

Hydrogenation reactions are used to change vegetable oils into margarine to be sold in supermarkets

Question 24

hydration reaction?

Answer 24

Alkenes also undergo addition reactions with steam in which an alcohol is formed. Since water is being added to the molecule it is also called a hydration reaction

The reaction is very important industrially for the production of alcohols and it occurs using the following conditions:

• Temperature of around 330ºC
• Pressure of 60 – 70 atm
• Concentrated phosphoric acid catalyst

Question 25

Alcohols?

Answer 25

Family of organic compounds that all contain the -OH functional group This is the group of atoms responsible for their chemical properties and reactions

Question 26

Ethanol?

Answer 26

* Ethanol (C2H5OH) is one of the most important alcohols * It is the type of alcohol found in alcoholic drinks such as wine and beer * It is also used as fuel for cars and as a solvent * Alcohols burn in excess oxygen and produce CO2 and H2O * Ethanol undergoes combustion: * CH3CH2OH + 3O2 → 2CO2 + 3H2O

Question 27

The manufacture of ethanol?

Answer 27

There are two methods used to manufacture ethanol: * The hydration of ethene with steam * The fermentation of glucose

Question 28

Hydration of ethene

Answer 28

* A mixture of ethene and steam is passed over a hot catalyst of phosphoric acid at a temperature of 330°C * The pressure is used 60-70 atmosphere * The gaseous ethanol is then condensed into a liquid for use A water molecule adds across the C=C in the hydration of ethene to produce ethanol (picture description)

Question 29

Fermentation of glucose

Answer 29

* Sugar or starch is dissolved in water and yeast is added * The mixture is then fermented between 15 and 35°C with the absence of oxygen for a few days * Yeast contains enzymes that break down starch or sugar to glucose * If the temperature is too low the reaction rate will be too slow and if it is too high the enzymes will become denatured * The yeast respire anaerobically using the glucose to form ethanol and carbon dioxide: * C6H12O6 + Enzymes → 2CO2 + 2C2H5OH * The yeast are killed off once the concentration of alcohol reaches around 15%, hence the reaction vessel is emptied and the process is started again * This is the reason that ethanol production by fermentation is a batch process

Question 30

Comparing Methods of Ethanol Production (table)

Answer 30

Question 31

Carboxylic acids?

Answer 31

* These are a homologous series of organic compounds that all contain the same functional group: –COOH * They are colourless liquids which are weakly acidic and have typical acidic properties * They react with alkaline solutions, turn blue litmus red and form salts called ethanoates

Question 32

Ethanoic Acid

Answer 32

* Ethanoic acid is a typically weak acid and dissociates slightly in water, producing a mildly acidic solution * The equilibrium lies far to the left during ionization: * CH3COOH ⇌ H+ + CH3COO- * Ethanoic acid reacts with the more reactive metals, hydroxides and carbonates

Question 33

Reactions of ethanoic acid with metals

Answer 33

In the reaction with metals a metal salt and hydrogen gas are produced For example in reaction with magnesium the salt magnesium ethanoate is formed: 2CH3COOH + Mg → (CH3COO)2Mg + H2

Question 34

Reactions of ethanoic acid with hydroxides

Answer 34

In the reaction with hydroxides a salt and water are formed in a neutralisation reaction For example in reaction with potassium hydroxide the salt potassium ethanoate is formed: CH3COOH + KOH → CH3COOK + H2O

Question 35

Reactions of ethanoic acid with carbonates

Answer 35

In the reaction with carbonates a metal salt, water and carbon dioxide gas are produced For example in reaction with potassium carbonate the salt potassium ethanoate is formed: 2CH3COOH + K2CO3 → 2CH3COOK + H2O + CO2

Question 36

Two ways to make carboxylic acids

Answer 36

**Oxidation by fermentation** **Oxidation with potassium manganate (VII)**

Question 37

Oxidation by fermentation

Answer 37

* The microbial oxidation of ethanol will produce a weak solution of vinegar (ethanoic acid). * This occurs when a bottle of wine is opened as bacteria in the air (acetobacter) will use atmospheric oxygen from air to oxidise the ethanol in the wine: * C2H5OH + O2 → CH3COOH + H2O * The acidic, vinegary taste of wine which has been left open for several days is due to the presence of ethanoic acid

Question 38

Oxidation with potassium manganate (VII)

Answer 38

* Alcohols can also be oxidised to carboxylic acids by heating with acidified potassium manganate (VII) * The heating is performed under reflux which involves heating the reaction mixture in a vessel with a condenser attached to the top * The condenser prevents the volatile alcohol from escaping the reaction vessel as alcohols have low boiling points Diagram showing the experimental setup for the oxidation with K2MnO4 using reflux apparatus

Question 39

Making esters?

Answer 39

* Alcohols and carboxylic acids react to make esters in esterification reactions * Esters are compounds with the functional group R-COO-R * Esters are sweet-smelling oily liquids used in food flavourings and perfumes * Ethanoic acid will react with ethanol in the presence of concentrated sulfuric acid (catalyst) to form ethyl ethanoate: * CH3COOH + C2H5OH → CH3COOC2H5 + H2O

Question 40

Naming esters

Answer 40

* An ester is made from an alcohol and carboxylic acid * The first part of the name indicates the length of the carbon chain in the alcohol, and it ends with the letters ‘- yl’ * The second part of the name indicates the length of the carbon chain in the carboxylic acid, and it ends with the letters ‘- oate’ * g. the ester formed from pentanol and butanoic acid is called pentyl butanoate *