Alkenes (Module 1)



Alkenes (Module 1)

a) state the general formula of alkenes

b) know and identify the functional group in alkenes

c) understand that alkenes are much more reactive than alkanes and explain why this is so

d) appreciate that alkenes are not used as fuels but as building blocks to produce other chemicals

e) describe the chemical reactions of alkenes (with conditions)

i) combustion (include colour of flame) ii) halogenation

iii) oxidation (hot and cold) iv) hydration v) hydrohalogenation

vi) hydrogenation (include production of trans-fat which are harmful)

and give equations for each chemical reaction

f) state the name of the mechanism by which alkenes use and describe the mechanism using ethene and bromine as an example

g) define Markovnikov’s Rule and apply it to the prediction of products for the reaction of hydrogen halides and UNsymmetrical alkenes

Alkenes are a family of hydrocarbons (compounds containing carbon and hydrogen only) containing ONE carbon-carbon double bond. They contain a multiple bond and thus are called UNSATURATED molecules. The first two are:

|ethene |C2H4 |

|propene |C3H6 |

You can work out the formula of any of them using the general formula: CnH2n

The functional group is the double bond C=C

Reactivity of alkenes vs alkanes

Alkenes have an electron rich centre i.e. partially negative region, the double bond or Π bond. Alkanes do not have either a partially positive or negative region as the difference in electronegativies between carbon and hydrogen atoms are small, resulting in the bonds being non-polar. Therefore alkenes are susceptible to electrophilic attack but alkanes are NOT susceptible to either electrophilic or nucleophilic attack. Hence alkenes are more reactive than alkanes.

Since alkenes are reactive compounds, they are NOT used as fuels like alkanes but as building blocks in the petro-chemical industry.

Reactions of alkenes

1. Combustion

With oxygen or air to carbon dioxide and water (in excess oxygen) if oxygen is limited, carbon monoxide and water are the products

e.g. C2H4 + 3O2 (2CO2 + 2H2O alkenes burn with a smoky yellow flame

2. Addition (all unsaturated compounds undergo addition reactions)

a) Hydrogenation

The hydrogenation of ethene

Ethene reacts with hydrogen in the presence of a finely divided nickel catalyst at a temperature of about 150°C. Ethane is produced.

[pic][pic]

Some margarine is made by hydrogenating carbon-carbon double bonds in animal or vegetable fats and oils. You can recognise the presence of this in foods because the ingredients list will include words showing that it contains "hydrogenated vegetable oils" or "hydrogenated fats".

Animal and vegetable fats and oils are similar molecules, differing in their melting points. If the compound is a solid at room temperature, you usually call it a fat. If it is a liquid, it is often described as an oil. Their melting points are largely determined by the presence of carbon-carbon double bonds in the molecule. The higher the number of carbon-carbon double bonds, the lower the melting point.

If there aren't any carbon-carbon double bonds, the substance is said to be saturated. Molecules of this sort are usually solid at room temperature. If there is only one carbon-carbon double bond in each of the hydrocarbon chains, it is called a mono-unsaturated fat (or mono-unsaturated oil, if it is an oil at room temperature.

If there are two or more carbon-carbon double bonds in each chain, then it is said to be polyunsaturated. However, there are possible health benefits in eating mono-unsaturated or polyunsaturated fats or oils rather than saturated ones - so you wouldn't want to remove all the carbon-carbon double bonds.

There are some probable health risks from eating hydrogenated fats or oils. Consumers are becoming more aware of this, and manufacturers are increasingly finding alternative ways of converting oils into spreadable solids. One of the problems arises from the hydrogenation process.

The double bonds in unsaturated fats and oils tend to have the groups around them arranged in the "cis" form.

The relatively high temperatures used in the hydrogenation process tend to flip some of the carbon-carbon double bonds into the "trans" form. If these particular bonds aren't hydrogenated during the process, they will still be present in the final margarine in molecules of trans fats.

The consumption of trans fats has been shown to increase cholesterol levels (particularly of the more harmful LDL form) - leading to an increased risk of heart disease.

b) THE HALOGENATION OF ALKENES

Ethene and chlorine or bromine or iodine

In each case you get an addition reaction. For example, bromine adds to give 1,2-dibromoethane.

[pic][pic]

The reaction with bromine happens at room temperature. If you have a gaseous alkene like ethene, you can bubble it through either pure liquid bromine or a solution of bromine in an organic solvent like tetrachloromethane. The reddish-brown bromine is decolourised as it reacts with the alkene.

Using bromine water as a test for the presence of a multiple bond

If you shake an alkene with bromine water (or bubble a gaseous alkene through bromine water), the solution becomes colourless. Alkenes decolourise bromine water.

[pic]

c) ALKENES and HYDROGEN HALIDES

Addition to symmetrical alkenes

All alkenes undergo addition reactions with the hydrogen halides. A hydrogen atom joins to one of the carbon atoms originally in the double bond, and a halogen atom to the other.

For example, with ethene and hydrogen chloride, you get chloroethane:

[pic][pic]

With but-2-ene you get 2-chlorobutane:

[pic]

Conditions

The alkenes react with gaseous hydrogen halides at room temperature. If the alkene is also a gas, you can simply mix the gases. If the alkene is a liquid, you can bubble the hydrogen halide through the liquid.

Addition to unsymmetrical alkenes

Orientation of addition

If HCl adds to an unsymmetrical alkene like propene, there are two possible ways it could add. However, in practice, there is only one major product.

[pic]

This is in line with Markovnikov's Rule which says:

When a compound HX is added to an unsymmetrical alkene, the hydrogen becomes attached to the carbon with the greater number of hydrogen atoms attached to it already.

In this case, the hydrogen becomes attached to the CH2 group, because the CH2 group has 2 hydrogen atoms and the CH group only has one hydrogen atom attached.

d) Oxidation of alkenes

Alkenes react with potassium manganate(VII) solution in the cold. The colour change depends on whether the potassium manganate(VII) is used under acidic or alkaline conditions.

If the potassium manganate(VII) solution is acidified with dilute sulphuric acid, the purple solution becomes colourless.

If the potassium manganate(VII) solution is made slightly alkaline (often by adding sodium carbonate solution), the purple solution first becomes dark green and then produces a dark brown precipitate.

Manganate(VII) ions are a strong oxidising agent, and in the first instance oxidise ethene to ethane-1,2-diol (old name: ethylene glycol).

:

• [pic]

NB Unsaturated compounds also react with hot, concentrated potassium manganate VII. This can be used to determine the position of a double bond in a compound. Below is the general form of an alkene

[pic]

The hot, conc. acidified potassium manganate(VII) solution oxidises the alkene by breaking the carbon-carbon double bond and replacing it with two carbon-oxygen double bonds.

[pic]

In conclusion, wherever a double bond is in a compound, it would be broken and C=O bond would formed at EACH carbon atom of the double bond.

This would result in either aldehydes or ketones being produced.

e) THE DIRECT HYDRATION OF ALKENES

Ethanol can be produced in the lab by reacting ethene with concentrated sulphuric acid at 170 °C. In industry, it is prepared by reacting ethene with steam at 300 °C and 60-70 atm using phosphoric acid as a catalyst.

[pic]

Alkenes react via the electrophilic addition mechanism

Using ethene and bromine as an example:-

Note: The arrows show the movement of the electrons towards a positively charged region.

In the first stage of the reaction, one of the bromine atoms becomes attached to both carbon atoms, with the positive charge being found on the bromine atom. A bromonium ion is formed.

[pic]

The bromonium ion is then attacked from the back by a bromide ion formed in a nearby reaction.

[pic]

Worksheet

1 a)

[pic]

Circle one functional group in the molecule above.

b) Draw the structural formula when retinol reacts with excess bromine in tetrachloromethane

c) Describe what would be observed in the reaction

………………………………………………………………………………………………………………………………………………………….

d) Calculate the molar mass of retinol (H = 1, O = 16, C = 12)

…………………………………………………………………………….

e) Using just a portion of the retinol molecule

[pic]

Show the mechanism of the reaction of retinol and bromine.

2.

[pic]

3. Why would saturated fats be a greater health risk than unsaturated fats?

………………………………………………………………………………………………………………………………………………………………………………….……………………………………………………

4. Using Markovnikov’s Rule, deduce the structural formulae of the major products of the reactions below:-

a) propene + HCl

b) pent-1-ene + Br2

c) but-2-ene + HBr

5. Show the mechanism of the reaction of propene + HCl[pic]

................
................

In order to avoid copyright disputes, this page is only a partial summary.

Google Online Preview   Download