Topic: Alcohols & Carboxylic Acids

Tricks for remembering the names of organic molecules

A good way to remember the names of organic molecules is to make up a silly mnemonic where the first letter of each word matches the first letter of the organic molecules. For example the first 10 alkanes in order are , Methane, Ethane, Propane, Butane, Pentane, Hexane, Heptane, Octane, Nonane and Decane. These can be memorised with “Many elephants prefer blue pinapples. However hungry orangutans never do.” This isn’t a very good mnemonic, but it is the one I made up for myself when I was at school, and you are best making up your own. The sillier, smellier and more colourful the better.

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-1-ene:

4:29 (Triple only) know that alcohols contain the functional group −OH

The member of the homologous series called Alcohols have names which end in “ol”. Examples are methanol, ethanol and propanol.

Alcohols all contain an -OH functional group attached to a hydrocarbon chain.

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