GCSE OCR Gateway Organic Chemistry C6.2c Addition reactions

GCSE OCR Gateway Organic Chemistry C6.2c Addition reactions

C6.2c To be able to predict the formulae and structures of products of reactions of the first four and other given members of the homologous series of alkanes, alkenes and alcohols

Reactions to include combustion; addition of bromine and hydrogen across a double bond; oxidation of alcohols to carboxylic acids using potassium manganate(VII)

Addition reactions of the alkenes

Addition reactions in alkenes occur across the carbon—carbon double bond.

In the addition reactions, essentially what happens is that one of the two bonds in the double bond breaks since one of the two is weaker than the other.

The reactant splits in two and each atom bonds to the two carbon atoms of the double bond.

The conditions are usually sufficient to set up a reaction in which the weaker bond breaks. 

a)    Bromination

You need to use neat bromine at room temperature

CH₂=CH₂(g)    +      Br₂(l)      ➞        CH₂Br CH₂Br(l)

                                 Ethene                                            1,2–dibromoethane

There is a colour change from orange bromine to colourless product.

This reaction is used as a test for the presence of a double bond in a hydrocarbon.

The reaction looks like this if we use displayed formulae:

b)    Hydrogenation

You need to use gaseous hydrogen in the presence of a finely divided nickel (Ni) catalyst at 300^oC.

CH₂=CH₂(g)    +      H₂(l)      ➞        CH₂H CH₂H(l)

                           ethene                                                 ethane

There is a change from an unsaturated molecule to the saturated colourless ethane.

This reaction is used to turn vegetable oil into solid margarine.

The reaction looks like this if we use displayed formulae:

Now you can substitute any double bonded molecule for the ethene and the same reaction will happen. 

The next blog looks at the oxidation of alcohols. 

GCSE OCR Gateway Organic Chemistry C6.2c Combustion reactions

GCSE OCR Gateway Organic Chemistry C6.2c Combustion reactions

C6.2c To be able to predict the formulae and structures of products of reactions of the first four and other given members of the homologous series of alkanes, alkenes and alcohols

Reactions to include combustion; addition of bromine and hydrogen across a double bond; oxidation of alcohols to carboxylic acids using potassium manganate(VII)

1.    Combustion of Alkanes

Alkanes burn completely in pure oxygen to form just water and carbon dioxide.

This is true of all alkanes. 

For example:

C₃H₈ (g)  +   5O₂ (g)   ⟶   3CO₂ (g)   +   4H₂O (l)

In the exact molar proportions (e.g. 10cm³ propane and 50 cm³ oxygen), this mixture of gaseous propane and pure oxygen will explode with some violence.  This property of explosive violence is true of all the gaseous alkanes with the correct molar proportions of oxygen.

You can tell an alkane is burning completely because it will burn with a pale blue flame.

The effect is seen when you open the air hole of a Bunsen burner and the methane flame changes from yellow to blue (See the photo below:)

You can find help to balance alkane combustion equations from my blog here

Incomplete combustion more often occurs in air since there is insufficient oxygen for complete combustion. 

The alkane will burn with a yellow flame because some hydrocarbon is converted into incandescent carbon, and that gives the flame its yellow colour.

For the past century and a half the combustion of alkane hydrocarbons has been the major fuel for transport whether octane based fuel in the internal combustion engine or diesel in the diesel engine or kerosine in jet engines. 

This irresponsible burning of valuable carbon based molecules cannot continue indefinitely because the earth has only a limited resource of such fuels.

The time is fast approaching when burning these fossil fuels for transport will change to other forms of propulsion whether hydrogen powered vehicles or electric powered engines. 

2.    Combustion of Alkenes

Alkenes contain a higher ratio of carbon to hydrogen than do alkanes so we would expect alkene to burn with a yellow flame incompletely to form carbon, carbon dioxide and water.  Here is an example of incomplete combustion of an alkene

C₃H₆ (g)  +   3½O₂ (g)   ⟶   C(s)     +      2CO₂ (g)   +   3H₂O (l)

Air does not contain enough oxygen and even pure oxygen is not probably not enough. 

If an alkene were to burn completely then here is the equation for the combustion of propene:

C₃H₆ (g)  +   4½O₂ (g)   ⟶   3CO₂ (g)   +   3H₂O (l)

Note that to balance the oxygen you will need to accept half a mole of oxygen in your equation or double all molar stoichiometric values as below.

2C₃H₆ (g)  +   9O₂ (g)   ⟶   6CO₂ (g)   +   6H₂O (l)

3.    Combustion of Alcohols

Alcohols burn completely in air with a pale blue flame to produce water vapour and carbon dioxide. 

You have probably seen this effect if you have seen a Christmas pudding set alight through the combustion of a liqueur or whisky of 40% strength or greater. 

Ethanol is the alcohol involved and it easily burns completely in air.

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

As the carbon chain increases the alcohol becomes more difficult to burn completely and eventually the alcohol changes from a liquid to a waxy solid. 

Alcohols up to octanol (C₈H₁₇OH(l)) will burn in air but they burn incompletely with yellow not blue flames. 

Brazil adds the alcohol ethanol at 10% of its petrol fuel to supplement its use.

In my next post, I’m going to talking about the addition reactions of alkenes with bromine and hydrogen.

GCSE OCR Gateway Organic Chemistry C6.2a-b Homologous Series

GCSE OCR Gateway Organic Chemistry C6.2a-b Homologous Series

C6.2a To be able to recognize functional groups and identify members of the same homologous series to include alkanes, alkenes, alcohols and carboxylic acids

C6.2b To be able to name and draw the structural formulae, using fully displayed formulae, of the first four members of the straight chain alkanes, alkenes, alcohols and carboxylic acids

What is a homologous series?

A homologous series is a series of organic compounds where one organic compound differs from the member next to it by a “CH₂” group. 

Each member of the series is a homologue of the other members that means that the series can be described by a general formula.

So for the alkanes that general formula is C_nH_2n+2 , for the alkenes C_nH_2n for the alcohols C_nH_2n+1OH and for the carboxylic acids C_nH_2n+1.COOH where n represents the number of carbon atoms in the particular molecule.

However, you have to ask to what extent these terms are useful today given the fact that they are of limited help in identifying any particular molecule because a general formula C_nH_2n can also describe a cyclic alkane and a large alkene can have the double bond in several different positions. 

The general formula tells us nothing about where that double bond might be situated on the carbon chain.  For example an alkene

C_nH_2n C₄H₈ could be CH₃CH₂CH=CH₂  or CH₃CH=CHCH₃ so the general formula or molecular formula tells you nothing about the structural differences. 

It is better to examine individual hydrocarbons and their derivative’s names to find out the structural information that is very useful.  Take but-2-ene :

First but- tells us that the molecule contains four carbon atoms and that due to the –ene suffix it has 8 hydrogen atoms since it is an alkene. i.e. C₄H₈.

So what does the -2- tell us? The -2- in but-2-ene tells us that the double bond starts on the second carbon atom of the chain, that is, it is the CH₃CH=CHCH₃ above.  But but-1-ene would tell us that the double bond starts on the first carbon atom in the chain that is CH₃CH₂CH=CH₂ above.  But-1-ene and but-2-ene are structural isomers: same molecular formulae but different arrangement of the atoms in the molecule.

The tables below show the names, molecular formulae, structural formulae and displayed formulae of the first four members of each of these homologous series: alkanes, alkenes, alcohols and carboxylic acids.

1. Alkane Homologous Series

Let’s now look at the alkanes up to alkane number four with four carbon atoms butane:

2. Alkene Homologous Series

The following table shows the first four alkenes and includes the butene isomers:

There is no methene because for an alkene to possess a double bond the molecule requires a minimum of two carbon atoms.

3. Alcohol Homologous Series

This table shows the first four straight chain alcohols in the homologous series,

4. Carboxylic acid Homologous Series 

Lastly let’s look at the carboxylic acid homologous series:

Make a note that in constructing the name of the acid from its displayed formula the carbon of the acid group is counted together with the carbon atoms in the carbon chain.

My next post will examine some of the reactions of these compounds.