Alkanes and alkenes
The An atom, or group of atoms, that determines the main chemical properties of an organic compound. present in an Compounds that contain carbon atoms, joined by covalent bonds to other atoms (including other carbon atoms). determines the compound鈥檚 typical reactions. Saturated hydrocarbon. A compound of hydrogen and carbon only, with no C=C bonds. and Unsaturated hydrocarbon with a double bond between the carbon atoms. have different functional groups, so:
- both undergo Burning in a plentiful supply of oxygen or air. Complete combustion of a hydrocarbon produces water vapour and carbon dioxide., but
- only alkenes can undergo Type of reaction in which two substances react together to form one new substance.
Complete combustion
The complete combustion of alkanes and alkenes involves the The gain of oxygen, or loss of electrons, by a substance during a chemical reaction. of A compound that contains hydrogen and carbon only.. In both cases, carbon dioxide and water are produced. For example:
ethane + oxygen 鈫 carbon dioxide + water
C2H6(g) + 3陆O2(g) 鈫 2CO2(g) + 3H2O(l)
ethene + oxygen 鈫 carbon dioxide + water
C2H4(g) + 3O2(g) 鈫 2CO2(g) + 2H2O(l)
Notice that more A collection of two or more atoms held together by chemical bonds. of oxygen are needed to balance the equation for ethane, because more water molecules are formed.
Question
Explain why alkanes and alkenes are oxidised during complete combustion
The alkanes and alkenes react with oxygen. Their carbon atoms and hydrogen atoms gain oxygen.
Addition reactions of alkenes
With bromine
The An atom, or group of atoms, that determines the main chemical properties of an organic compound., C=C, allows alkenes to undergo addition reactions. For example, ethene reacts with bromine to form 1,2-dibromoethane:
CH2=CH2 + Br2 鈫 CH2BrCH2Br
It is easier to see what happens using displayed formulae:
The reaction is an 鈥榓ddition鈥 reaction because one molecule combines with another molecule, forming one larger molecule and no other A substance formed in a chemical reaction..
Question
Propene, CH3CH=CH2, reacts with bromine. Predict the structure of the product formed.
The structure will be: CH3CHBrCH2Br.
Question
But-1-ene, CH3CH2CH=CH2, reacts with bromine. Predict the structure of the product formed.
The structure will be: CH3CH2CHBrCH2Br.
With hydrogen
Alkenes undergo addition reactions with hydrogen, in the presence of a nickel A substance that changes the rate of a chemical reaction without being changed by the reaction itself., to form alkanes. For example:
ethene + hydrogen 鈫 ethane
C2H4(g) + H2(g) 鈫 C2H6(g)
Notice that:
- there is only one product
- an unsaturated compound has formed a saturated compound
Testing for alkanes and alkenes
An orange-brown solution of bromine dissolved in water, called bromine water, is used to distinguish between alkanes and alkenes:
- there is no change when bromine water is mixed with an alkane
- the bromine water turns colourless when it is mixed with an alkene
