Friday, June 6, 2008
ALKANES & ALKENES
@ 9:41 PM
These are what we covered in chem for today. Those who missed the lesson, hope this helps. These are suppose to be filled into the notes given for Organic Chem. (:
ALKANES
General Formula of alkanes :
1 carbon - Methane (CH4) [Molecular Mass - 16]
2 carbons - Ethane (C2H6) [Molecular Mass - 30]
3 carbons - Propane (C3H8) [Molecular Mass - 44]
4 carbons - Butane (C4H10) [Molecular Mass - 72]
5 carbons - Hexane (C6H14) [Molecular Mass - 86]
Formula : n carbons - (C(n)H(n+2))
All alkanes consist of only single bonds.
All alkanes are saturated because each carbon atom is bonded to a maximum of 4 atoms.
Alkanes are the simplest members of the hydrocarbon family:
-Contains only hydrogen and carbon atoms.
-Only have single C-C bonds.
Members of the alkane homologous series will have:
-Similar chemical properties.
-A series of physical properties as a result of an increase in the size & mass of the molecules.
Physical Properties Of Alkanes
CH4 Methane - B.P. -161 (Mr 16)
C2H6 Ethane - B.P. -89 (Mr 30)
C3H8 Propane B.P. -42 (Mr 44)
Physical Properties of Alkanes
-Short chain lengths ( 1 < n < 4 ) are gases at room temperature.
-Medium chain lengths ( 4 < n < 17 ) are liquids at room temperature.
-Long chain lengths ( n > 17 ) are solids at room temperature.
>>> as the number of carbons increase, the boiling point of alkanes increases.
1. Physical Properties
-They are covalent molecules with intermolecular forces between the atoms. Hence they have a low boiling point.
-The boiling point increase as the number of carbon atoms in the molecule increases.
-As the molecules get larger and larger, they become less flammable as the molecules burns less easily when heat is applied.
1.Physical Properties
-Most alkanes are liquids or gases at room temperature.
-They are insoluble in water.
-The compounds become more viscous as the longer molecules get tangled together.
-The density increases when the molecules are bigger.
Chemical Reactions of Alkanes
-In general, alkanes are very unreactive, as their C-C and C-H bonds are very strong.
-They do not react with most chemicals, like acids, bases and oxidising agents.
-However, under certain conditions, alkanes undergo 2 types of chemical reactions:
1)Combustion
2)Substitution
Chemical Reactions of Alkanes
1)Complete Combustion - exothermic reaction
All alkanes burn in pletiful supply of air to form carbon dioxide and steam:
CH4(g) + 202(g) -> CO2(g) + 2H20(g)
Methane + Oxygen -> Carbon dioxide + steam
Incomplete combustion
>>When there isn't enough oxygen present, this can lead to the formation of carbon(soot) or carbon monoxide.
>>Carbon monoxide(CO) is poisonous. If you breath in enough carbon monoxide, you will die from internal suffocation as it blocks oxygen from being absorbed into the red blood cells in the haemoglobin.
2)Substitution reaction
Reaction with Chlorine
Alkanes react with chlorine in the presence of light. For example, Methane reacts with chlorine.
CH4(g) + Cl2(g) -> CH3Cl(g) + HCl(g)
Methane(g) + chlorine(g) -> chloromethane(g) + hydrogen chloride(g)
A substitution reaction is one in which an atom or a group of atoms replace other atoms in a molecule.
The substitution reactions of alkanes with chlorine will not take place in darkness. Light is needed to start the reaction. The light energy is used to break the covalent bonds in the chlorine molecules to produce chlorine atoms.
CH3Cl - Chloromethane
CH2Cl2 - Dichloromethane
CH3Cl3 - Trichloromethane
CCl4 – tetrachloromethane
ALKENES
-All names end with ~ene.
-Alkenes is another homologous series of unsaturated hydrocarbons with the general formula of C(n)H(2n).
-Alkenes are called unsaturated hydrocarbons because they have ONE double carbon-carbon covalent bond in them (C=C).
-They are more reactive than alkanes.
2 carbons - Ethene (C2H4)
3 carbons - Propene (C3H6)
4 carbons - Butene (C4H8)
5 carbons - Pentene (C5H10)
6 carbons - Hexene (C6H12)
-Alkenes form a homologous series; all members in the series have similar molecular formulae and chemical properties.
-Their physical properties changes slightly as they become bigger; eg. The boiling points increases with increasing size.
-They are different from the successive member by a -CH2- group.
The two structures are of the SAME PROPENE. Only the position of the double bond is different.
These are called ISOMERS – Organic compounds with the same chemical properties but different chemical structures.
CRACKING
What is cracking?
-Big organic molecules like crude oil as they do not burn easily and have a high density (high).
-Hence they are not very useful and have to be processed into smaller units by cracking.
-Most of the useful fractions are smaller in molecular mass. Eg. LPG, petrol and jet fuel.
How is cracking done?
-Cracking is done by passing the big alkane molecule in the form of vaporized oil over a solid catalyst at a high temperature.
-Big oil molecules will break up into small oil molecules; this mixture is then separated by fractional distillation.
Uses of cracking
1. Big hydrocarbon molecules are broken into smaller hydrocarbon molecules for use as motor fuels.
-The smaller molecules burns more easily, is lighter and more environment-friendly.
2. Cracking is a way of making alkenes. A lot of ethene used for making of plastic (polyethene) and ethanol is produced in this manner.
C20H42 -> C12H26 + C8H16
C6H14 -> C4H10 + C2H4
3. Cracking is used to make hydrogen gas for Haber process and used as fuels in rockets.
C12H26 -> C12H24 + H2
C22H46 -> C12H24 + 5C2H4 + H2
Chemical Properties of alkenes
1. Combustion : Alkenes burn in air to form CO2 and H2O.
CxHy + (x+y)/2 O2 -> xCO2 + xH2O
C2H4 + 3O2 -> 2CO2 + 2H2O
C3H6 + 9/2O2 -> 3CO2 + 3H2O
C4H8 + 6O2 -> 4CO2 + 4H2O
-If the combustion is incomplete (insufficient supply of oxygen) the combustion will produce and would form carbon dioxide, water vapour, soot and carbon monoxide.
-In general, alkenes give a more sooty flame compared to alkanes as alkenes have a higher % of carbon per unit mass and because of the C=C bond.
2. Addition reactions
-Alkenes having one C=C as a functional group is quite reactive.
-The double bond undergoes addition reactions with small molecules like bromine, chlorine, hydrogen and water etc.
a)Bromination
Bromine reacts with ethane to form a colourless compound, reaction occurs at the C=C bond.
Chemical equation: C2H4(aq) + Br2(aq) -> C2H4Br2(aq)
-Ethene would rapidly decolourise liquid bromine to form dibromethane but not ethane.
-This is also used as a test for alkenes and other unsaturated hydrocarbons.
-Ethene is an unsaturated compound as it can add on other atoms;
-All alkenes are unsaturated.
-Alkanes which cannot have other atoms added is saturated.
-The reaction can be used as a test for alkenes.
Observations:
-Alkenes react with reddish brown bromine and decolourises it quickly.
-If the alkene is a liquid, the 2 are mixed together in a test tube and shaken.
-If the alkene is a gas, bubble the gas through aqueous bromine.
3. Hydrogenation
-Alkenes react with hydrogen to produce alkanes
-Conditions: Nickel is used as a catalyst at 180°C
-Hydrogenation is used in the food industry to convert vegetable oil to margarine
How is margarine formed?
-Unsaturated vegetable oils (alkanes) can be partially saturated by hydrogenation to produce a soft solid known as margarine.
-Hydrogenation increases the molecular mass of the vegetable oils.
-This increases the strength of the intermolecular attractions and hence increases its melting point. Thus liquid vegetable oil changes into a solid.
-Vegetable oils are polysaturated and are healthier.
Poly = large molecules
unsaturated = alkenes = contain C=C bond
4. Hydration
-Is also called catalytic addition of steam
-Production of alcohol from alkene
Conditions:
1. react with steam at 300°C
2. using phosphoric acid as catalyst
-A molecule of water is added to the alkene molecule
Example 1: C2H4 + H2O -> C2H5OH (ethanol)
Ethanol is used as solvent for perfume, cologne etc.
Example 2: C3H6 + H2O -> C3H7OH (propanol)
Well that’s all. (: Hope it helps.