Alkanes and Alkenes
what is the displayed formula
A displayed formula shows all the bonds in a molecule as individual lines. Each line represents a pair of shared electrons in a covalent bond. This shows a model of butane, together with its displayed formula:
what is a functional group
A functional group is an atom or group of atoms that determines the chemical properties of a compound. All compounds in the same homologous series have the same functional group. For example, the functional group for alcohols is the -OH group and that for alkenes is C=C
what is a homologous series
A homologous series is a series of compounds with similar chemical properties because they have the same functional group (what makes a particular compound differ from that of an alkane for example). Each member differs from the next by one -CH2-
What is the molecular formula
A molecular formula counts the actual number of each type of atom present in a molecule. For example, the molecular formula of butane is C4H10 and the molecular formula of ethene is C2H4 The molecular formula tells you nothing about the ways the atoms are joined together, the structure.
What is the structural formula
A structural formula shows how the atoms in a molecule are joined together. There are two ways of representing structural formula: they can be drawn as a displayed formula (full structural formula) or be written out in condensed form by omitting all the carbon-carbon single bonds and carbon-hydrogen single bonds. For example: CH3CH2CH3
What is substitution reaction between alkenes and alkanes?
A subsitution react occurs when an atom or group of atoms is replaced by a different atom or group of atoms. for example bromine: This is between a alkane and a halogen
substitution reaction of alkanes with halogens
Alkanes react with halogens in the presence of ultraviolet radiation (UV). For example from sunlight. A hydrogen atom in the alkane is replaced by a halogen atom. This is known as a substitution reaction because one atom has been replaced by a different one. A mixture of methane and bromine has is orange because of the presence of bromine. If it is exposed to sunlight, it loses its colour and a mixture of bromoethane and hydrogen bromide gases is formed. (A very similar reaction happens with ethane and chlorine but you get a mixture of chloroethane and hydrogen chloride)
why are alkanes saturated and alkenes are unsaturated
Alkenes are unsaturated compounds because they contain a C=C bond. A saturated compound contains C-C bonds whereas an unsaturated compound contains one or more double or triple C-C bonds.
what is the general formula for alkenes
Alkenes have the general formula CnH2n. the number of hydrogen atoms is twice the number of carbon atoms. This means that all alkenes will have the same empirical formula: CH2 The alkene with 11 carbons will have 22 hydrogen atoms in the molecular formula C11H22
What is the empirical formula?
An empirical formula tells you the simplest whole number ratio of the atoms in a compound. It can be calculated from experimental data. Sometimes the empirical formula is the same as the molecular formula. For example, the empirical formula of methane is CH4, which is the same as its molecular formula. However, the empirical formula of ethane is CH3 whereas its molecular formula is C2H6.
addition reaction of bromine
Bromine adds to alkenes without any need for heat, light or a catalyst. The reaction is often carried out using bromine water. Eg: with ethene You know a reaction has happened because bromine water is orange but when the alkenes react the bromine water reacts and turns colourless
what is the trend in chemical properties in alkanes
Chemical properties are dependent on the functional group and bonding within the molecules. Because the alkanes have C-C bond and C-H they are going to behave in the same way. These are strong covalent bonds, and therefore alkanes are fairly unreactive.
Combustion of alkanes and alkenes
Combustion is just another way of saying burniing and involves a reaction with oxygen Combusion of hydrocarbons in excess oxygen gives rise to carbon dioxide and water, together with the release of a large amount of heat energy. For example: Propane burning C3H8 (g) + 5O2 (g) 3CO2 (g) + 4H20 (l) Butene burning C4H8 (g) + 6O2 (g) -> 4CO2 (g) + 4H2O (l) Propane is frequently used in camping stoves
structural isomers for alkenes? where is there no structural isomer?
Ethene = CH2=CH2 doesn't have any isomers Propene = CH3CH=CH3 doesn't have any isomers C4H8 = butene - you can vary the position of the double bond
what is a addition reaction
In an addition reaction something is added to a molecule without taking anything away. Alkenes undergo addition reactions. For example, ethene reacts with bromine
Combustion reaction of alkenes
In common with all hydrocarbons, alkenes burn in air or oxygen to give carbon dioxide and water. For example: C2H4(g) + 2O2 (g) -> 2CO2 (g) + 2H2O(l)
what is the general formula of alkanes?
In the case of the alkanes, if there are n carbons, there are 2n + 2 hydrogens. The general formula = CnH2n+2 So for example, if there are 15 carbons 2x15 +2 = 32 = C15H32 The reason that the alkanes and other homologous series can be described by a general formula is that each member differs from the next by a -CH2- unit.
how to know what compound is what?
One part of the name tells you how many carbon atoms there are in the longest chain, another part tells you whether there are carbon-carbon double bonds. You can remember this by Monkeys Eat Peanut Butter you can tell if its an alkane or alkene from the ending letters: Alkene: 'ene' - propene Alkane 'ane'- methane
Example of branched butane
The longest chain in the molecule has four carbon atoms (but) with no double bonds (ane). The name is based on butane. There is a methyl group branching from the number 2 carbon. Therefore, the compound is called 2-methylbutane
Where do the double bonds go?
The rule is that you number from the end which produces the smaller numbers in the name.
What are structural isomers?
Structural isomers are molecues with the same molecular formula but different structural formula. Example: structural formula od C5H12 if you had 5 carbon atoms and 12 hydrogen atoms you can see that you can join them up in one or more ways. The differnet structures are known as isomers. All have the molecular formula of C5H12 but different structures:
addition reaction with alkenes
The C=C double bond is the functional group. The functional group determines the chemical properties of a compound. alkenes are much more reactive than alkanes. Alkenes undergo addition reactions. Part of the double bond breaks to become single C-C bond and the electrons are used to join other atoms onto the two carbon atoms.
What are alkanes?
The alkanes are the simplest family of hydrocarbon. Hydrocarbons are compounds that contain carbon and hydrogen only. Many of these alkanes are used as fuels. Methane is the major component of natural gas. Ethane and propane are also present in small quantities in natural gas and are important constitutions of petroleum gases from crude oil separation.
isomers for alkanes
The alkanes can all be described as straight-chain alkanes or having an unbranched chain. There are also isomers of butane and propane that have branched chains
homologous series of alkanes
The alkanes form a homologous series. A homologous series is a series of compounds that: - Have the same functional group - Have similar chemical properties - Show a trend (gradation) in physical properties - Can be described by the same general formula - Differ from the next by a -CH2- unit The alkanes form the simplest homologous series. The alkanes do not really have a functional group as they just contain single C-C and C-H bonds, which are the basis of all other organic compounds.
what happens to the boiling point as you go through the homologous series of alkanes? why?
The first four alkanes are gases at room temperature (25°C). all the other alkanes (At gcse) are liquids.# The molecules of the members of a homologous series increase in size in a regular way and you can see that the boiling points also increase in a regular way. As the molecules become bigger, the strength of the inter-molecular forces of attraction between them increases. This means that more energy has to be put in to break the attractions between one molecule and its neighbours. One effect of this is that the boiling point increase in a regular way
what is the link between the molecular and empirical formula?
The molecular formula is a multiple of the empirical formula. For example if a compound has an empirical formula of CH2, the molecular formula could be C2H3, C3H6, C4H8etc. the ratio of C:H must be always the same as in the empirical formula.
what are side branches?
The name of a molecule is always based on the longest chain you can find in it. The position of the side chain is shown by numbering the carbon atoms from the end of the longest chain which produces the smaller numbers in the name, exactly as before with the carbon-carbon double bonds in alkanes.
what are hydrocarbons
The simplest organic compounds are hydrocarbons. These are molecules that contain carbon and hydrogen ONLY. The carbon atoms are joined together with single, double or triple bonds. Carbon atoms are joined to hydrogen atoms by single bonds.
what does the term organic mean? what is an organic compound?
The term 'organic' is a chemical compound that contains Carbon and hydrogen. Organic compounds can exist as chains, branched chains or rings of carbon atoms with hydrogens attached.
What is the general formula
There are many different families of organic compounds, known as homologous series. Examples of homologous series are alkanes, alkenes and alcohols Members of a homologous series can be represented using a general formula
isomer example of butene
There are two straight-chain isomers with the formula C4H8
physical properties of alkenes
These are very similar to the ones of alkanes. The first four alkenes are gases at room temperature. They are gases up to C4H8 and then the next dozen or so are liquids.
difference between branched and non-branched chain
Unbranched chain -> straight chain Branched -> not straight
How to draw a structural formula?
You can simplify the formula by writing, for example, CH3 or CH2 instead of showing all the carbon-hydrogen bonds. The structural formula for butane can be shown as: CH3CH2CH2CH3 or CH3-CH2-CH2-CH3 = C4H10 You can also write out all the carbon-hydrogen bonds like this: These both show how the atoms in the molecule are joined together.
test to see if its saturated or a unsaturated compound
if you have an unknown organic compound you can use bromine water to see if its contains a double bond (alkene) or not (alkane) alkanes: do not react with the bromine water, so stays orange because does not contain double bonds alkenes: turns colourless because the bromine water has reacted with the alkene's double bonds breaking them and adding bromine atoms into the alkene. them and adding bromine atoms into the alkene. Part of the carbon-carbon double bonds breaks and two bromine atoms add across the bond.
what are alkenes?
unsaturated hydrocarbons. which contain a carbon-carbon double bond. The C=C bond is the functional group of the alkenes.
