Alkanes & Alkenes
Alkanes
Properties of Alkanes
Reaction of Alkanes
CH3Cl + Cl2 > CH2Cl2 + HCl
CH2Cl2 + Cl2 > CHCl3 + HCl
CHCl3 + Cl2 > CCl4 + HCl
The sp2 hybrids form shorter, fatter and arrange themselves as far apart from each other as possible – 120o. Hence being planar.
Speak to Crowe
Isomerism
E/Z isomerism (Cahn-Ingold-Prelog nomenclature)
Reactions of Alkenes
Any compound containing a C=C bond will decolourise bromine water
Alkanes
Properties of Alkanes
- Single Carbon-Carbon covalent bonds
- Saturated molecules
- CnH2n+2
- Simple molecules held together by weak London forces
- Branching occurs frequently
- Alkanes with more than 4 carbons begin to exhibit structural isomerism
Reaction of Alkanes
- Alkanes are unreactive due to C-C and C-H bonds having very high bond enthalpies
- Non polar as the electronegativity of C and H are so similar
- Combustion
- Complete combustion with a plentiful supply of oxygen
- Incomplete combustion with a limited supply of oxygen
- This is harmful as carbon monoxide combines irreversibly with haemoglobin to form carboxyhaemoglobin which prevents oxygen transportation
- CO is colourless and odourless and very toxic
- Reactions with bromine and chlorine
- Methane and chlorine do not react at all in the dark but in the sunlight they do react
- UV must be present for the bonds to break
CH3Cl + Cl2 > CH2Cl2 + HCl
CH2Cl2 + Cl2 > CHCl3 + HCl
CHCl3 + Cl2 > CCl4 + HCl
- The mechanism
- Referred to as free radical substitution refer to separate page
- Fractional Distillation
- Crude oil is a mixture of compounds, mainly alkanes
- A fraction is a group or range of hydrocarbons with similar carbon lengths that are piped off in the same place
- There is a graduation of heat meaning the gases will condense at different temperatures
- As molecular mass of alkanes increases
- Their boiling point increases
- Harder to ignite
- More dense
- More viscous (thicker)
- Unsaturated hydrocarbons produced via cracking
- Functional group of alkenes is carbon-carbon double bond
- More reactive than alkanes
- CnH2n
- Melting and boiling increase as the carbon atoms increase
- Insoluble in water
The sp2 hybrids form shorter, fatter and arrange themselves as far apart from each other as possible – 120o. Hence being planar.
Speak to Crowe
Isomerism
- Movement around a double bond is restricted
- This means that isomers can be formed around a carbon double bond
- Two different types of stereoisomers; E/Z and optical
- Cis-trans isomers occur if there are two different groups on each carbon in the double bond
- Cis = on this side
- Trans = on opposite side / across
E/Z isomerism (Cahn-Ingold-Prelog nomenclature)
- Some molecules are too complex to use cis-trans
- If there are different groups on each carbon in a double bond
- E = OppositE
- Z = together
- The atoms / molecules attached to the carbon are assigned priorities
- The priorities are decided based on the molar mass of the immediately attached atom
- The higher the molar mass is the atom branch with priority
- If the higher priority groups are on opposite sides then it is an E isomer
- If the higher priority groups are on the same side then it is an Z isomer
- However cis- does not always mean Z:
- Both carbons have a methyl group attached, this is therefore clearly cis
- However bromine has a larger molar mass then carbon – this means bromine is a priority on the right and carbon is the priority on the right
- This is therefore an E isomer so we would name it:
Reactions of Alkenes
- A C=C bond is an area of high electron density meaning alkenes are much more reactive
- Pi bonds are much more exposed so readily break during addition reactions
- Combustion
- Carbon dioxide and water will be formed
- The higher the ratio of carbon to hydrogen in the hydrocarbon the sootier the flame
- Addition of hydrogen (catalytic hydrogenation)
- Important in the manufacture of margarine, by adding hydrogen we can increase the molecular mass of oils straighten their chains, this thickens the oil
- 423 K
- Catalyst – Ni (finely divided – greater surface area)
- Addition of X2
- Chlorine and bromine add rapidly at room temperature
- Products are dichloroalkanes or dibromoalkanes
- Fluorine reacts explosively and iodine reacts slowly
- Can be used as a test for unsaturated hydrocarbons if the bromine water is decolourised
Any compound containing a C=C bond will decolourise bromine water
- Addition of a hydrogen halide
- Similar to addition of a halogen to an alkene
- The gaseous hydrogen halide adds readily at room temperature
- If both reactants are gaseous then they react
- If the alkene is liquid then the hydrogen halide needs to be bubbled through it
- Alkenes also react with concentrated HCl(aq) and HBr(ap)
- Two isomers can be formed
- Hydration reactions
- Alcohols are produced when alkenes react with steam
- A catalyst is required for this reaction – H3PO4
- Used to produce ethanol from ethane
- Two structural isomers can be formed
- Addition reaction
- Two reactants join together to form one product
- Carbocation
- When the double bond is broken in a molecule in preparation for halidation
- The carbon atom is a positively charged cat ion
- Nucleophile
- An atom that will donate electrons in a pair
- Attracted to something electron deficientAlkanes & Alkenes
Alkanes
Properties of Alkanes - Single Carbon-Carbon covalent bonds
- Saturated molecules
- CnH2n+2
- Simple molecules held together by weak London forces
- Branching occurs frequently
- Alkanes with more than 4 carbons begin to exhibit structural isomerism
- In an alkane the bonds are hybridised into sp3 orbitals. This is a more stable electron structure (to promote an electron from the s orbital to the p orbital). The bonds are also sigma bonds as the orbitals overlap head on with the s orbital of the hydrogen
Reaction of Alkanes - Alkanes are unreactive due to C-C and C-H bonds having very high bond enthalpies
- Non polar as the electronegativity of C and H are so similar
- Combustion
- Complete combustion with a plentiful supply of oxygen
- CH4 + 2O2 > CO2 + 2H2O
- Incomplete combustion with a limited supply of oxygen
- 2C2H6 + 5O2 > 4CO + 6H2O
- This is harmful as carbon monoxide combines irreversibly with haemoglobin to form carboxyhaemoglobin which prevents oxygen transportation
- CO is colourless and odourless and very toxic
- Reactions with bromine and chlorine
- Methane and chlorine do not react at all in the dark but in the sunlight they do react
- UV must be present for the bonds to break
- CH4 + Cl2 > CH3Cl + HCl
CH3Cl + Cl2 > CH2Cl2 + HCl
CH2Cl2 + Cl2 > CHCl3 + HCl
CHCl3 + Cl2 > CCl4 + HCl - The mechanism
- Referred to as free radical substitution refer to separate page
- Fractional Distillation
- Crude oil is a mixture of compounds, mainly alkanes
- A fraction is a group or range of hydrocarbons with similar carbon lengths that are piped off in the same place
- There is a graduation of heat meaning the gases will condense at different temperatures
- As molecular mass of alkanes increases
- Their boiling point increases
- Harder to ignite
- More dense
- More viscous (thicker)
- Alkenes
- Unsaturated hydrocarbons produced via cracking
- Functional group of alkenes is carbon-carbon double bond
- More reactive than alkanes
- CnH2n
- Melting and boiling increase as the carbon atoms increase
- Insoluble in water
- In an alkene, the carbon atom doesn’t have enough unpaired electrons to form the required number of bonds, it needs to promote one of the 2s2 pair into the empty 2pz orbital. The carbon is in an excited state. In alkenes each carbon is only joining to three other atoms and so they only hybridise three. They use the 2s electron and two of the p orbitals, alkenes use an sp2 hybrids and the 2pz orbital goes unused.
The sp2 hybrids form shorter, fatter and arrange themselves as far apart from each other as possible – 120o. Hence being planar.
Speak to Crowe
Isomerism - Movement around a double bond is restricted
- This means that isomers can be formed around a carbon double bond
- Stereoisomers have the same molecular and structural formula but different arrangement of the atoms in the space
- Two different types of stereoisomers; E/Z and optical
- Cis-trans isomers are stereoisomers that occur due to restricted rotation around a carbon carbon double bond
- Cis-trans isomers occur if there are two different groups on each carbon in the double bond
- Cis = on this side
- Trans = on opposite side / across
-
E/Z isomerism (Cahn-Ingold-Prelog nomenclature) - Some molecules are too complex to use cis-trans
- If there are different groups on each carbon in a double bond
- E = OppositE
- Z = together
- The atoms / molecules attached to the carbon are assigned priorities
- The priorities are decided based on the molar mass of the immediately attached atom
- The higher the molar mass is the atom branch with priority
- If the higher priority groups are on opposite sides then it is an E isomer
- If the higher priority groups are on the same side then it is an Z isomer
- However cis- does not always mean Z:
-
- Both carbons have a methyl group attached, this is therefore clearly cis
- However bromine has a larger molar mass then carbon – this means bromine is a priority on the right and carbon is the priority on the right
- This is therefore an E isomer so we would name it:
- (E) – but-2-ene
Reactions of Alkenes - A C=C bond is an area of high electron density meaning alkenes are much more reactive
- Pi bonds are much more exposed so readily break during addition reactions
- Combustion
- Carbon dioxide and water will be formed
- The higher the ratio of carbon to hydrogen in the hydrocarbon the sootier the flame
- C3H6 + 4.5O2 > 3H2O + 3CO2
- Addition of hydrogen (catalytic hydrogenation)
- Important in the manufacture of margarine, by adding hydrogen we can increase the molecular mass of oils straighten their chains, this thickens the oil
- 423 K
- Catalyst – Ni (finely divided – greater surface area)
- C3H6 + H2 > C3H8
- Addition of X2
- Chlorine and bromine add rapidly at room temperature
- Products are dichloroalkanes or dibromoalkanes
- Fluorine reacts explosively and iodine reacts slowly
- Can be used as a test for unsaturated hydrocarbons if the bromine water is decolourised
- C3H6 + Br2 > C3H6Br
Any compound containing a C=C bond will decolourise bromine water - Addition of a hydrogen halide
- Similar to addition of a halogen to an alkene
- The gaseous hydrogen halide adds readily at room temperature
- If both reactants are gaseous then they react
- If the alkene is liquid then the hydrogen halide needs to be bubbled through it
- Alkenes also react with concentrated HCl(aq) and HBr(ap)
- Two isomers can be formed
- C2H4 + HBr > C2H5Br
- Hydration reactions
- Alcohols are produced when alkenes react with steam
- A catalyst is required for this reaction – H3PO4
- Used to produce ethanol from ethane
- Two structural isomers can be formed
-
- Addition reaction
- Two reactants join together to form one product
- Carbocation
- When the double bond is broken in a molecule in preparation for halidation
- The carbon atom is a positively charged cat ion
- Nucleophile
- An atom that will donate electrons in a pair
- Attracted to something electron deficient