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Chemistry > Organic Compounds - Alkanes,enes,acids > Flashcards

Organic Compounds - Alkanes,enes,acids Flashcards

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1
Q

Alkanes

A

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

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2
Q

Saturated hydrocarbons?

A

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

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3
Q

Reactivity of alkanes?

A

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.

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4
Q

Substitution Reaction of Alkanes with Halogens

A

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

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5
Q

Describe the bonding in alkanes

A
  • Saturated (only contain single covalent bonds).
  • Each carbon atom forms four covalent bonds.
  • Contain C-C and C-H covalent bonds.
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6
Q

What is required for alkanes to react with chlorine?

A

UV radiation

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7
Q

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

A

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

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8
Q

Describe how alkenes can be manufactured

A
  • 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
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9
Q

catalytic cracking?

A

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

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10
Q

Distinguishing between alkanes and alkenes

A
  • 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:
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11
Q

What’s the difference between saturated and unsaturated compounds?

A

Saturated - only contain single covalent bonds.

Unsaturated - contain at least one double bond.

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12
Q

How can you distinguish between saturated and unsaturated compounds?

A

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

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13
Q

Why do alkenes decolourise bromine water?

A

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

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14
Q

Alkenes?

A

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

General formula:

CnH2n

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15
Q

Catalytic cracking explanation in Alkenes (PICTURE):

A

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

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16
Q

Addition Reaction?

A

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
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17
Q

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

A

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

18
Q

In terms of alkenes, what is an addition reaction?

A

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

19
Q

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

A

Ethene + Bromine → 1,2-dibromoethane

C2H4 + Br2 → C2H4Br2

20
Q

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

A

Propene + hydrogen → Propane C3H6 + H2 → C3H8

21
Q

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

A

Ethene + steam → ethanol C2H4 + H2O → C2H5OH

22
Q

Addition Polymerisation

A
  • 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
23
Q

Hydrogenation reactions?

A

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

24
Q

hydration reaction?

A

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
25
Q

Alcohols?

A

Family of organic compounds that all contain the -OH functional group

This is the group of atoms responsible for their chemical properties and reactions

26
Q

Ethanol?

A
  • 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
27
Q

The manufacture of ethanol?

A

There are two methods used to manufacture ethanol:

  • The hydration of ethene with steam
  • The fermentation of glucose
28
Q

Hydration of ethene

A
  • 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)

29
Q

Fermentation of glucose

A
  • 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
30
Q

Comparing Methods of Ethanol Production (table)

A
31
Q

Carboxylic acids?

A
  • 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
32
Q

Ethanoic Acid

A
  • 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
33
Q

Reactions of ethanoic acid with metals

A

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

34
Q

Reactions of ethanoic acid with hydroxides

A

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

35
Q

Reactions of ethanoic acid with carbonates

A

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

36
Q

Two ways to make carboxylic acids

A

Oxidation by fermentation

Oxidation with potassium manganate (VII)

37
Q

Oxidation by fermentation

A
  • 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
38
Q

Oxidation with potassium manganate (VII)

A
  • 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

39
Q

Making esters?

A
  • 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
40
Q

Naming esters

A
  • 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
    *

Chemistry (5 decks)

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