1.1 Hydrocarbons  (Page 5/6)

Naming the alkenes

Give the IUPAC name for each of the following alkenes:

1. CH ${}_2$ CHCH ${}_2$ CH ${}_2$ CH ${}_3$
2. CH ${}_3$ CHCHCH ${}_3$

3. 

The properties of the alkenes

The properties of the alkenes are very similar to those of the alkanes, except that the alkenes are more reactive because they are unsaturated. As with the alkanes, compounds that have four or less carbon atoms are gases at room temperature, while those with five or more carbon atoms are liquids.

Reactions of the alkenes

Alkenes can undergo addition reactions because they are unsaturated. They readily react with hydrogen, water and the halogens. The double bond is broken and a single, saturated bond is formed. A new group is then added to one or both of the carbon atoms that previously made up the double bond. The following are some examples:

1. Hydrogenation reactions A catalyst such as platinum is normally needed for these reactions $H_{2}C=C{H}_2+H_2\to H_{3}C-C{H}_3$ ( [link] )

   

2. Halogenation reactions $C{H}_2=C{H}_2+HBr\to C{H}_3-C{H}_2-Br$ ( [link] )

   

3. The formation of alcohols $C{H}_2=C{H}_2+H_{2}O\to C{H}_3-C{H}_2-OH$ ( [link] )

   

The alkenes

1. Give the IUPAC name for each of the following organic compounds:

   1. 

   2. CH ${}_3$ CHCH ${}_2$
2. Refer to the data table below which shows the melting point and boiling point for a number of different organic compounds.

   Formula	Name	Melting point ( ${}^0$ C)	Boiling point ( ${}^0$ C)
   C ${}_4$ H ${}_{10}$	Butane	-138	-0.5
   C ${}_5$ H ${}_{12}$	Pentane	-130	36
   C ${}_6$ H ${}_{14}$	Hexane	-95	69
   C ${}_4$ H ${}_8$	Butene	-185	-6
   C ${}_5$ H ${}_{10}$	Pentene	-138	30
   C ${}_6$ H ${}_{12}$	Hexene	-140	63

   1. At room temperature (approx. 25 ${}^0$ C), which of the organic compounds in the table are:
      1. gases
      2. liquids
   2. In the alkanes...
      1. Describe what happens to the melting point and boiling point as the number of carbon atoms in the compound increases.
      2. Explain why this is the case.
   3. If you look at an alkane and an alkene that have the same number of carbon atoms...
      1. How do their melting points and boiling points compare?
      2. Can you explain why their melting points and boiling points are different?
   4. Which of the compounds, hexane or hexene, is more reactive? Explain your answer.
3. The following reaction takes place: $C{H}_{3}CHC{H}_2+H_2\to C{H}_{3}C{H}_{2}C{H}_3$
   1. Give the name of the organic compound in the reactants.
   2. What is the name of the product?
   3. What type of reaction is this?
   4. Which compound in the reaction is a saturated hydrocarbon?

The alkynes

In the alkynes, there is at least one triple bond between two of the carbon atoms. They are unsaturated compounds and are therefore highly reactive. Their general formula is C ${}_n$ H ${}_{2n-2}$ . The simplest alkyne is ethyne ( [link] ), also known as acetylene. Many of the alkynes are used to synthesise other chemical products.

Naming the alkynes

The same rules will apply as for the alkanes and alkenes, except that the suffix of the name will now be -yne.

Give the IUPAC name for the following compound:

1. Identify the functional group

   There is a triple bond between two of the carbon atoms, so this compound is an alkyne. The suffix will be -yne. The triple bond is at the second carbon, so the suffix will in fact be 2-yne.

2. Find the longest carbon chain and give the compound the correct prefix

   If we count the carbons in a straight line, there are six. The prefix of the compound's name will be 'hex'.

3. Number the carbons in the longest chain

   In this example, you will need to number the carbons from right to left so that the triple bond is between carbon atoms with the lowest numbers.

4. Look for any branched groups, name them and show the number of the carbon atom to which the group is attached

   There is a methyl (CH ${}_3$ ) group attached to the fifth carbon (remember we have numbered the carbon atoms from right to left).

5. Combine the elements of the name into a single word in the following order: branched groups; prefix; name ending according to the functional group and its position along the longest carbon chain.

   If we follow this order, the name of the compound is 5-methyl-hex-2-yne .

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The alkynes

Give the IUPAC name for each of the following organic compounds.

1. 

2. C ${}_2$ H ${}_2$
3. CH ${}_3$ CH ${}_2$ CCH