Topic: Alcohols & Carboxylic Acids

4:02 understand how to represent organic molecules using empirical formulae, molecular formulae, general formulae, structural formulae and displayed formulae

The molecular formula shows the actual number of atoms of each element in a molecule.

The empirical formula shows the simplest whole number ratio of atoms present in a compound. So the molecular formula is a multiple of the empirical formula.

The general formula shows the relationship between the number of atoms of one element to another within a molecule. Members of a homologous series share the same general formula. The general formula for alkanes is CnH2n+2 and the general formula for alkenes is CnH2n.

A structural formula shows how the atoms in a molecule are joined together.

The displayed formula is a full structural formula which shows all the bonds in a molecule as individual lines.

 

The terms above are demonstrated with the example of butane.

Image result for butane

  • Displayed formula:
  • Molecular formula: C₄H₁₀
  • Empirical formula: C₂H₅
  • General formula (alkanes): CnH2n+2
  • Structural formula: CH₃ – CH₂ – CH₂ – CH₃

 

The terms above are demonstrated with the example of ethene, which contains a double bond.

Image result for ethene

  • Displayed formula:
  • Molecular formula: C₂H₄
  • Empirical formula: CH₂
  • General formula (alkenes): CnH2n
  • Structural formula: CH₂ = CH₂

4:03 know what is meant by the terms homologous series, functional group and isomerism

A functional group is an atom or a group of atoms that determine the chemical properties of a compound.

For example the functional group of an alcohol is the -OH group and that of alkenes is the C=C carbon to carbon double bond.

 

A Homologous series is a group of substances with:

  • the same general formula
  • similar chemical properties because they have the same functional group
  • a trend (graduation) in physical properties

 

 

Isomers are molecules with the same molecular formula but with a different structure.

4:04 understand how to name compounds relevant to this specification using the rules of International Union of Pure and Applied Chemistry (IUPAC) nomenclature. Students will be expected to name compounds containing up to six carbon atoms

The names of organic molecules are based on the number of carbon atoms in the longest chain. This chain is the longest consecutive line of carbon atoms, even if this line bends. 

The name is based on the number of carbon atoms in the longest chain
1 Meth-
2 Eth-
3 Prop-
4 But-
5 Pent-
6 Hex-
7 Hept-
8 Oct-
9 Non-
10 Dec-

Hydrocarbons are molecules which contain only hydrogen and carbon.

 

Naming straight-chain alkanes

The simplest hydrocarbons are alkanes. They contain only single bonds, and have “-ane” in the name.

For example, the displayed formula of ethane (C₂H₆) is:

The name “ethane” contains “eth-” because there are 2 carbon atoms in the longest chain, and the name contains “-ane” because the molecule only has single bonds so is an alkane.

 

Another example is pentane (C₅H₁₂) which has the displayed formula:

The name “pentane” contains “pent-” because there are 5 carbon atoms in the longest chain, and the name contains “-ane” because the molecule only has single bonds, so is an alkane.

 

Remember, it does not matter if the longest consecutive line of carbons bends around. For example the displayed formula below still shows a very normal molecule of pentane (5 carbons in a row). Pentane is not normally drawn with the longest chain of carbons bent around because it could be confusing.

 

You might also see the bonds drawn at angles. Don’t worry, the displayed formula below is still pentane, as can be seen by the fact there are 5 carbon atoms in the longest chain, surrounded by hydrogen atoms bonded to the carbon atoms by single bonds.

Image result for displayed formula pentane

 

A shorter way to express the detailed structure of an organic molecule is the structural formula. The structural formula for pentane is CH₃-CH₂-CH₂-CH₂-CH₃, which tells us the same information about the molecule as does the displayed formula, without the hassle of having to draw all the bonds or all the hydrogen atoms.

 

Naming straight-chain alkenes

Another simple group of hydrocarbons is the alkenes. They contain a carbon-to-carbon double bond, which also means they have two fewer hydrogen atoms than their corresponding alkane. An alkene has “-ene” in its name.

For example, the displayed formula for ethene (C₂H₄) is:

ethene has 2 carbon atoms and 4 hydrogen atoms

 

and the displayed formula of propene (C₃H₆) is:

propene has 3 carbon atoms and 6 hydrogen atoms

 

With longer alkene molecules the double bond might appear in different locations of the carbon chain, so the name needs to be a little bit more complicated to be able to describe these differences clearly. A number is added in the middle of the name to indicate at which carbon the double bond starts.

So the displayed formula of pent-1-ene is:

 

and this is the displayed formula of pent-2-ene:

 

However, take care that when counting which carbon has the double bond. The numbers start from the end that produces the smallest numbers in the name. For example, this is the displayed formula for pent-1-ene again, but just drawn the other way round. It is still pent-1-ene (you can’t get pent-4-ene):

 

Naming straight-chain alcohols

We get the same pattern all over again with the group of organic molecules called alcohols, which are recognised by an -OH functional group. For example here is the displayed formula for ethanol, which has 2 carbon atoms in the longest chain:

Image result for displayed formula ethanol

 

and here is the displayed formula for butanol:

Image result for displayed formula butanol

 

Summary of naming simple straight-chain organic molecules

The following table summarises the naming of some of the straight-chain alkanes, alkenes and alcohols, giving a name and a molecular formula for each:

Carbons in longest chainAlkanesAlkenesAlcohols
1methane, CH₄-methanol, CH₄O
2ethane, C₂H₆ethene, C₂H₄ethanol, C₂H₆O
3propane, C₃H₈propene, C₃H₆propanol, C₃H₈O
4butane, C₄H₁₀propane, C₄H₈butanol, C₄H₁₀O

 

Naming branched alkanes and alkenes

The naming conventions for organic molecules cover more than the straight chain molecules. Branched molecules are named depending on the number of carbon atoms in the branch. A branch with 1 carbon is called “methyl” and a branch with 2 carbons is called “ethyl”. This is similar to the conventions covered above, plus the “-yl-” bit just says it is a branch.

For example, this is the displayed formula for 2-methyl hexane:

In the name 2-methyl hexane, the number 2 indicates that when counting along the longest carbon chain the methyl branch comes off the second carbon atom. The “methyl” bit of the name says there is one branch of 1 carbon. The “hex” bit of the name says the longest consecutive chain of carbon atoms is 6. The “ane” bit says the molecule has only single bonds.

 

When counting along the carbon atoms of the longest chain to work out the name, the numbering of carbon atoms starts from the end nearest to the branch. Another way to put this is that the name is given such that the numbers in the name are as low as possible. For example, here is the displayed formula for 4-ethyl octane:

 

Another example, this time with 2 methyl branches coming off the second and third carbons of the chain, is 2,3-dimethyl hexane. The “di” in the name indicates there are two methyl groups. This is the same way in which “di” indicates there are two oxygen atoms in carbon dioxide.

 

Another example of how the naming convention works for branches is 2,2-dimethyl hexane:

 

This naming of branches also applies to alkenes. Here is the displayed formula of 4-methylpent-2-ene:

4:05 understand how to write the possible structural and displayed formulae of an organic molecule given its molecular formula

The molecular formula describes the actual number of each type of each atom in a molecule.

For example, a molecular formula of C₆H₁₂ tells us that in each molecule there are 6 carbon atoms and 12 hydrogen atoms.

However, the molecular formula tells us nothing about how those atoms are arranged. For example, it does not tell us if there any branches of carbon atoms coming off the main carbon-carbon chain, nor how long or how many there might be.

On the other hand, the structural formula and displayed formula of a molecule tell us clearly how the atoms are arranged in that molecule.

This means that if we are given a molecular formula only, there may be several possible structural and displayed formulae all of which could apply for that molecule.

When trying to work out possible structural or displayed formulae from a molecular formula there are several clues:

  • If the molecular formula only has carbon and hydrogen in it, then of course the structural formula will only have atoms of these 2 elements.
  • If the molecular formula has twice as many carbons as hydrogens (CnH2n) then the molecule is an alkene and has a double bond somewhere between 2 of the carbon atoms.
  • If the molecular formula has two more than twice as many carbons as hydrogens (CnH2n+2) then the molecule is an alkane and only has single bonds.
  • If the molecular formula has two more than twice as many carbons as hydrogens and also has an oxygen atom (CnH2n+2O), then the molecule is an alcohol, and somewhere in the displayed formula will be a carbon single-bonded to an oxygen which itself is then single-bonded to a hydrogen.

4:06 understand how to classify reactions of organic compounds as substitution, addition and combustion. Knowledge of reaction mechanisms is not required

In a substitution reaction an atom or group of atoms is replaced by a different atom or group of atoms. For example when ethane reacts with bromine gas one of the hydrogen atoms in ethane is substituted by one of the atoms of bromine from within the bromine molecule:

CH₃-CH₃         +         Br-Br         →         CH₃-CH₂Br         +         H-Br

ethane         +         bromine         →         bromoethane         +         hydrogen bromide

 

 

An addition reaction occurs when an atom or group of atoms is added to a molecule without taking anything away. For example when ethene reacts with bromine gas, the product is simply the addition of the two molecules:

CH₂=CH₂         +         Br-Br         →         CH₂Br-CH₂Br

Image result for ethene + bromine displayed formula

 

 

A combustion reaction is another way to say ‘burning’ and is a reaction with oxygen. Combustion of hydrocarbons with excess oxygen gives the products water and carbon dioxide, and also releases heat energy (exothermic reaction). Two examples the combustion of propane and the combustion of butene:

C₃H₈         +         5O₂         →         3CO₂         +         4H₂O

C₄H₈         +         6O₂         →         4CO₂         +         4H₂O

4:15 explain how the combustion of some impurities in hydrocarbon fuels results in the formation of sulfur dioxide

Fossil fuels such as coal, gas and oil are derived from crude oil.

These fuels are hydrocarbons, but also include impurities such as sulfur.

When the fuels are burned, sulfur dioxide is produced which can escape into the atmosphere:

S (s)         +         O₂ (g)         →         SO₂ (g)

 

4:16 understand how sulfur dioxide and oxides of nitrogen oxides contribute to acid rain

Acids formed in the atmosphere can fall as acid rain. This can be a major problem, killing trees and fish in lakes. The acid rain also corrodes limestone buildings and marble statues since these are both made of calcium carbonate (CaCO₃). Some metals such as iron are also attacked by acid rain.

 

Sulfur dioxide released into the atmosphere from the burning of fossil fuels can react with water and oxygen to make sulfuric acid (H₂SO₄):

2SO₂ (g)         +         2H₂O (l)         +         O₂ (g)         →         2H₂SO₄ (aq)

 

Also, if sulfur dioxide in the atmosphere reacts with just water, a weaker acid called sulfurous acid (H₂SO₃) is formed:

SO₂ (g)         +         H₂O (l)         →         H₂SO₃ (aq)

 

In car engines the temperature is high enough for the nitrogen in the air to react with oxygen to produce oxides of nitrogen, e.g:

N₂ (g)         +         O₂ (g)         →         NO₂ (g)

In the atmosphere these nitrogen oxides can produce nitric acid (HNO₃).

4:20 explain why alkanes are classified as saturated hydrocarbons

 

Saturated:   A molecule containing only single bonds between carbon atoms. For example, alkanes as described as saturated molecules.

Unsaturated:   A molecule containing a carbon-carbon double bond. For example, alkenes as described as unsaturated molecules.

 

4:21 understand how to draw the structural and displayed formulae for alkanes with up to five carbon atoms in the molecule, and to name the unbranched-chain isomers

The displayed formulae show all the atoms and bonds drawn out.

The molecular formulae just show the number of each type of atom in the molecule.

4:22 describe the reactions of alkanes with halogens in the presence of ultraviolet radiation, limited to mono-substitution knowledge of reaction mechanisms is not required

Alkanes react with bromine in the presence of UV light, e.g. sunlight.

A hydrogen atom in the alkane is replaced by a bromine atom.

This is known as substitution.

 

4:23 know that alkenes contain the functional group >C=C<

Alkenes are a homologous series of hydrocarbons which contain a carbon-carbon double bond. This double bond is shown in formulae as a double line.

The names of alkenes end with “ene”.

An example is ethene, the structural formula for which is CH₂ = CH₂

For a molecule with more than two carbon atoms, the position of the double bond within the molecule can vary as indicated by the name and the structural formula.

4:26 understand how to draw the structural and displayed formulae for alkenes with up to four carbon atoms in the molecule, and name the unbranched-chain isomers. Knowledge of cis/trans or E/Z notation is not required

The displayed formulae show all the atoms and bonds drawn out.

The molecular formulae just show the number of each type of atom in the molecule.

4:27 describe the reactions of alkenes with bromine, to produce dibromoalkanes

Alkenes react with bromine water. UV light is not required for this reaction.

The double bond is broken and the bromine atoms are added. This is an addition reaction.

During this reaction there is a colour change from orange to colourless.

For example:

This is how we can test for the presence of an alkene or another type of unsaturated molecule.

 

 

4:30 (Triple only) understand how to draw structural and displayed formulae for methanol, ethanol, propanol (propan-1-ol only) and butanol (butan-1-ol only), and name each compound, the names propanol and butanol are acceptable

Structural formula and displayed formula for methanol:

CH₃-OH

 

Structural formula and displayed formula for ethanol:

CH₃-CH₂-OH                    (or simply C₂H₅OH)

Image result for ethanol

 

Structural formula and displayed formula for propan-1-ol:

CH₃-CH₂-CH₂-OH

 

Structural formula and displayed formula for butan-1-ol:

CH₃-CH₂-CH₂-CH₂-OH

 

4:31 (Triple only) know that ethanol can be oxidised by: burning in air or oxygen (complete combustion), reaction with oxygen in the air to form ethanoic acid (microbial oxidation), heating with potassium dichromate(VI) in dilute sulfuric acid to form ethanoic acid

1) Ethanol can be oxidised by complete combustion. With excess oxygen the complete combustion of ethanol (C₂H₅OH) in air produces carbon dioxide and water:

C₂H₅OH (l)         +         3O₂ (g)         →         2CO₂ (g)         +         3H₂O (l)

 

2) Ethanol can be oxidised in air in the presence of microorganisms (‘microbial oxidation’) to form ethanoic acid (CH₃COOH).

 

3) Ethanol can be oxidised by heating with the oxidising agent potassium dichromate(VI) (K₂Cr₂O₇) in dilute sulfuric acid (H₂SO₄).

In the equation below, [O] means oxygen from an oxidising agent.

CH₃CH₂OH         +         2[O]         →         CH₃COOH         +         H₂O

This mixture starts orange but when the reaction happens turns green which indicates the presence of Cr³⁺ ions which are formed when the potassium dichromate(VI) is reduced.

4:32 (Triple only) know that ethanol can be manufactured by: 1) reacting ethene with steam in the presence of a phosphoric acid catalyst at a temperature of about 300⁰C and a pressure of about 60–70atm; and 2) the fermentation of glucose, in the absence of air, at an optimum temperature of about 30⁰C and using the enzymes in yeast

In the hydration of ethene, ethanol is made by passing ethene and steam over a catalyst.

Water is added to ethene, this is known as hydration.

Conditions

Catalyst: Phosphoric acid (H3PO4)

Temperature: 300°C

Pressure: 60 atm

 

 

Fermentation is the conversion of sugar, e.g. glucose into ethanol by enzymes from yeast.

 

Conditions

Catalyst: Zymase (enzyme found in yeast)

Temperature: 30°C  –  The process is carried out at low temperatures as not to denature the enzymes.

Other: Anaerobic (no oxygen present) – if oxygen were present, the yeast produce carbon dioxide and water instead of ethanol.

 

4:33 (Triple only) understand the reasons for fermentation, in the absence of air, and at an optimum temperature

In the production of ethanol the process of fermentation is carried out at a low temperature (30⁰-40⁰).

Above 40⁰ the enzymes would permanently lose their structure (denature).

At a temperature lower than 30⁰ the process would be too slow.

 

Fermentation is conducted in the absence of air. In the presence of air (aerobic conditions), enzymes in the yeast produce carbon dioxide and water instead of ethanol.

 

 

4:35 (Triple only) understand how to draw structural and displayed formulae for unbranched- chain carboxylic acids with up to four carbon atoms in the molecule, and name each compound

The four simplest carboxylic acids are methanoic acid, ethanoic acid, propanoic acid and butanoic acid.

 

Methanoic acid

Displayed formula:Image result for methanoic acid

Molecular formula: CH₂O₂

Structural formula: HCOOH

 

Ethanoic acid

Displayed formula:

Molecular formula: C₂H₄O₂

Structural formula: CH₃COOH

 

Propanoic acid

Displayed formula:Image result for propanoic acid

Molecular formula: C₃H₆O₂

Structural formula: CH₃CH₂COOH

 

Butanoic acid

Displayed formula:Image result for butanoic acid

Molecular formula: C₄H₈O₂

Structural formula: CH₃CH₂CH₂COOH

4:36 (Triple only) describe the reactions of aqueous solutions of carboxylic acids with metals and metal carbonates

Dilute carboxylic acids react with metals in the same way as other dilute acids (e.g. hydrochloric acid) only more slowly.

For example, dilute ethanoic acid reacts with magnesium with a lot of fizzing to produce a salt and hydrogen, leaving a colourless solution of magnesium ethanoate:

magnesium      +      ethanoic acid      →      magnesium ethanoate      +      hydrogen

Mg (s)         +         2CH₃COOH (aq)         →         (CH₃COO)₂Mg (aq)         +         H₂ (g)

 

Dilute carboxylic acids react with metal carbonates as they do with other acids, to give a salt, carbon dioxide and water.

For example, dilute ethanoic acid reacts with sodium carbonate with a lot of fizzing to produce a salt,  carbon dioxide and water, leaving a colourless solution of sodium ethanoate:

sodium carbonate      +      ethanoic acid      →      sodium ethanoate      +      carbon dioxide      +      water

Na₂CO₃ (s)         +         2CH₃COOH (aq)         →         2CH₃COONa (aq)         +         CO₂ (g)         +         H₂O (l)

 

As can be seen in the examples above the charge on the ethanoate ion is -1.

 

4:39 (Triple only) know that ethyl ethanoate is the ester produced when ethanol and ethanoic acid react in the presence of an acid catalyst

Ethyl ethanoate is the ester produced when ethanol and ethanoic acid react in the presence of an acid catalyst.

ethanoic acid           +           ethanol           ⇋           ethyl ethanoate           +           water

CH₃COOH (l)         +         CH₃CH₂OH (l)         ⇋         CH₃COOCH₂CH₃ (l)         +         H₂O (l)

The ethyl ethanoate produced is an ester.

The reaction is called esterification.

The reaction can also be described as a condensation reaction because water is made when two molecules join together.

4:41 (Triple only) understand how to write the structural and displayed formulae of an ester, given the name or formula of the alcohol and carboxylic acid from which it is formed and vice versa

To work out the structure of the ester formed when an alcohol reacts with a carboxylic acid, it is easiest to first draw the structures of alcohol and acid and then remove the H₂O to see what is left when the molecules join.

For example, propan-1-ol and ethanoic acid react together to form propyl ethanoate and water.

         propan-1-ol            +            ethanoic acid            →            propyl ethanoate            +            water

         CH₃CH₂CH₂OH (l)         +         CH₃COOH (l)         →         CH₃COOCH₂CH₂CH₃ (l)          +         H₂O (l)

 

The displayed formula for esters is typically written to show the part which came from the carboxylic acid on the left:

This is still called propyl ethanoate: the alcohol bit of the name (propyl) comes first and then the carboxylic acid bit (ethanoate), even though the displayed formula is typically written the other way around. Notice that the structural formulae for esters is also typically written with the carboxylic acid bit first, so propyl ethanoate is CH₃COOCH₂CH₂CH₃.

4:42 (Triple only) know that esters are volatile compounds with distinctive smells and are used as food flavourings and in perfumes

Esters are volatile compounds with distinctive smells. A volatile liquid is one that turns into to a vapour easily.

Esters are used as food flavourings and in perfumes.

Esters are often described as having a sweet, fruity smell. They typically smell of bananas, raspberries, pears or other fruit because esters occur in all these natural products.

4:43 (Triple only) practical: prepare a sample of an ester such as ethyl ethanoate

Heating a mixture of ethanoic acid and ethanol produces a liquid called ethyl ethanoate. A few drops of concentrated sulfuric acid must be added for the reaction to work. The sulfuric acid acts as a catalyst.

ethanoic acid           +           ethanol           ⇋           ethyl ethanoate           +           water

CH₃COOH (l)         +         CH₃CH₂OH (l)         ⇋         CH₃COOCH₂CH₃ (l)         +         H₂O (l)

The ethyl ethanoate produced is an ester.

The reaction is called esterification.

The reaction can also be described as a condensation reaction because water is made when two molecules join together.

Notice that the reaction is reversible. Pure reactants are used to maximise the yield of ethyl ethanoate. Pure ethanoic acid is called glacial ethanoic acid.

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Section 1: Principles of chemistry

      a) States of matter

      b) Atoms

      c) Atomic structure

     d) Relative formula masses and molar volumes of gases

     e) Chemical formulae and chemical equations

     f) Ionic compounds

     g) Covalent substances

     h) Metallic crystals

     i) Electrolysis

 Section 2: Chemistry of the elements

     a) The Periodic Table

     b) Group 1 elements: lithium, sodium and potassium

     c) Group 7 elements: chlorine, bromine and iodine

     d) Oxygen and oxides

     e) Hydrogen and water

     f) Reactivity series

     g) Tests for ions and gases

Section 3: Organic chemistry

     a) Introduction

     b) Alkanes

     c) Alkenes

     d) Ethanol

Section 4: Physical chemistry

     a) Acids, alkalis and salts

     b) Energetics

     c) Rates of reaction

     d) Equilibria

Section 5: Chemistry in industry

     a) Extraction and uses of metals

     b) Crude oil

     c) Synthetic polymers

     d) The industrial manufacture of chemicals