Download - Alkanes, alkenes &alkynes
ALKANES, ALKENES, ALKYNES
NOMENCLATURE, STRUCTURES AND ISOMERISM
Alkanes = CnH2n+2
Alkenes = CnH2n
Alkynes = CnH2n-2
ALKANES, ALKENES, ALKYNES AND CYCLOALKANES ARE HYDROCARBONS (COMPOUNDS CONTAINING ONLY CARBON AND HYDROGEN).
EACH OF THESE FORM A HOMOLOGOUS SERIES (A GROUP OF ORGANIC COMPOUNDS HAVING A COMMON GENERAL FORMULA/ OR IN WHICH EACH MEMBER FIFFERS FROM THE NEXT BY A –CH2)
THE HYDROCARBONS MAY BE SATURATED (CONTAINS ONLY SINGLE BONDS BETWEEN CARBON-CARBON ATOMS/ CARBON ATOMS BONDED TO THE MAXIMUM NUMBER OF HYDROGENS)
OR UNSATURATED (CONTAINS AT LEAST A DOUBLE BOND BETWEEN C-C ATOMS)
ALKANES: NOMENCLATURE
Also called paraffins.A group of saturated hydrocarbons with the
general formula Cn H2n+2 .They form a homologous series.Straight chain alkanes have their carbon
atoms bonded together to give a single chain
Alkanes may also be branched.
NAMING (GENERAL)
Hydrocarbon names are based on: 1) type, 2) # of carbons, 3) side chain type and position
1) name will end in -ane, -ene, or -yne2) the number of carbons is given by a “prefix”
1 meth- 2 eth- 3 prop- 4 but- 5 pent- 6 hex- 7 hept- 8 oct- 9 non- 10 dec-
Actually, all end in a, but a is dropped when next to a vowel. E.g. a 6 C alkene is hexene
Determine the longest continuous chain (not always straight) in the molecule. The base name of the hydrocarbon is the name of the longest chain.
IUPAC system
IUPAC SYSTEM
Name any chain branching off the longest chain as an alkyl group (e.g., methyl, ethyl etc)
The complete name of a branch requires a number that locates the branch on the longest chain.
Therefore number the chain in whichever direction gives the smaller number for all branches.
6. When two or more branches are identical, use prefixes (di-, tri-, etc.) (e.g. 2,4-dimethylhexane). Numbers are separated with commas. Prefixes are ignored when determining alphabetical order. (e.g. 2,3,5-trimethyl-4-propylheptane)
7. When identical groups are on the same carbon, repeat the number of this carbon in the name. (e.g. 2,2-dimethylhexane)
Where there are two or more different alkyl branches, the name of each branch, with its position number precedes the name. the branch names are placed in alphabetical order.
Alkenes and alkynes
Both groups are unsaturated hydrocarbons. Each group is a homologous series.The main chain is defined as the chain
containing the greatest number of double/tripple bonds
We number the position of the double/tripple bond so that it has the lowest numbers.
ALKENES
alkynes
Naming side chainsExample: name the following structure
CH3 CH2 C
CH2
CH2 C
CH2
CH3
CH3
CH3
Step 1 – Identify the correct functional group
Naming side chains
CH3 CH2 C
CH2
CH2 C
CH2
CH3
CH3
CH3
Step 2 - find the longest chain
Naming side chains
CH3 CH2 C
CH2
CH2 C
CH2
CH3
CH3
CH3
Step 3 - add the prefix naming the longest chain
Naming side chains
CH3 CH2 C
CH2
CH2 C
CH2
CH3
CH3
CH3
Step 4 - number the longest chain with the lowest number closest to the double bond
CH3 CH2 C2
CH21
CH23
C4
CH25
CH3
CH3
CH36
CH3 CH2 C
CH2
CH2 C
CH2
CH3
CH3
CH3
Naming side chains
Step 5 - add that number to the name
CH3 CH2 C2
CH21
CH23
C4
CH25
CH3
CH3
CH36
CH3 CH2 C
CH2
CH2 C
CH2
CH3
CH3
CH3
Naming side chains
ethyl
methyl
methylStep 6 - Name the side chains
CH3 CH2 C
CH2
CH2 C
CH2
CH3
CH3
CH3
CH3 CH2 C2
CH21
CH23
C4
CH25
CH3
CH3
CH36
Naming side chains
ethyl
methyl
methylStep 7 - Place the side chains in alphabetical order & name the compound
name the following
CH3 CH2CH CH3
CH2CH2
CH3
CH3 CH
CH
CH3
CH
CH3
CH2 CH2 CH3
CH2 CH3
CH3CH2CH CH CH CH2CH CH3
CH3
CH2CH3
CH3 CH3
CH3
CH2CH2
CH2CH2
CH2CH2
CH3
CH3
CHCH2
CH2CH
CH2CH2
CH3
CH3
CH3
CH2
CHCH2
CHCH2
CH2
CH3
CH2 CH3
CH2 CH CH C CH3CH3
CH3
CH3
1
2
3
4
CH3 CH2CH CH3
CH2CH2
CH3
CH3 CH
CH
CH3
CH
CH3
CH2 CH2 CH3
CH2 CH3
CH3CH2CH CH CH CH2CH CH3
CH3
CH2CH3
CH3 CH3
9 10
11
CH3 C CH CH CH3
CH2 CH2
CH3
CH3
CH CH
CH2
CH
CH3
CH3
CH3
CH2 C CCH2
CH3
CH3
ISOMERS
A GOOD TIME TO INTRODUCE ISOMERS (COMPOUNDS WITH THE SAME MOLECULAR FORMULA BUT DIFFERENT STRUCTURAL FORMULAE)
TRY THE FOLLOWING:
Reactions of alkanes & alkenes
We study three particular reaction cases:
SubstitutionAdditionEliminationCombustion
Reactions of alkanes
Substitution (of H, commonly by Cl or Br)Combustion (conversion to CO2 & H2O)
CombustionWhen alkanes are heated in a plentiful
supply of air, combustion occursAlkanes are energetically unstable with
respect to water and carbon dioxideThey only burn when they are in the
gaseous state Explain what happens when a candle burns!
2 C4H10(g) + 13 O2(g) 8 CO2(g) + 10 H2O(g)
2 C8H18(l) + 25 O2(g) 16 CO2(g) + 18 H2O(g)
SUBSTITUTION
Reactions with chlorineAlkanes only react with chlorine when a
mixture of the two is exposed to sunlight or ultraviolet light
The light provides the energy required to break the very strong bonds
This is an example of a substitution reaction
In the presence of light, or at high temperatures, alkanes react with halogens to form alkyl halides. Reaction with chlorine gives an alkyl chloride.
CH4(g) + Cl2(g) CH3Cl(g) + HCl(g)
Cracking
Cracking happens when alkanes are heated in the absence of air
The products of the cracking of long-chain hydrocarbons are shorter chain molecules
Ethane is cracked industrially to produce ethene
PHYSICAL PROPERTIES
Alkanes are non polar so they are insoluble in water but soluble in each other.
Low molecular alkanes are gases.Boiling points increase with increasing
chain length (molecular weight) for the first few members
Boiling points decrease with increasing number of branches.(Explain this in terms of Van der Waals’ forces and surface area.
Melting and boiling points increase with increased molecular weight (Methane bp.
-164°C, decane bp. 174°C)While boiling point decrease with chain
branching (decrease in surface area), melting
points increase· Alkanes are less dense than water and
swim on top of water
alkenes: preparation and reactions
Alkenes: Preparation and reactions
Two ways of making alkenes:1. Heat a concentrated solution of potasium
/sodium hydroxide in alcohol (alcoholic KOH) with a haloalkane (halogenoalkane)
This is dehydrohalogenation (removal of hydrogen and halogen)
2. Heat concentrated sulphuric acid with the alcohol- dehydration. THE ACID IS A DEHYDRATING AGENT
i) Dehydration of alcohols
conc. H2SO4R-CH2-CH2-OH R-CH=CH2 + H2O
ii) Dehydrohalogenation of haloalkanesNaOH/ethanolR-CH2-CH2-X reflux R-CH=CH2 + HX
NaOH can be replaced by KOH
LEARNERS MUST KNOW MAJOR PRODUCTS IN ALL CASES AND REACTION CONDITIONS
CH3CH2-CH-CH3OH
H+
H+
CH3CH=CH-CH3 + H2O
CH3CH2-CH=CH2 + H2O
2-butanol2-butenemajor product
1-butene
Dehydration of alcohols
Dehydrohalogenation of haloalkanes
CH3CH-CH-CH2
BrH HKOH CH3CH=CH-CH3 CH3CH2CH=CH2
alcoholreflux
2-bromobutane2-butene(major product) 1-butene
REACTIONS OF ALKENES (VIP)
Catalytic hydrogenation:- hydrogenation: addition of hydrogen
to a double bond and triple bond to yield saturated product.
- alkenes will combine with hydrogen in the present to catalyst to form alkanes.
C C H H C CH H
Pt or Pd25-90oC
- Plantinum (Pt) and palladium (Pd) – Catalysts - Pt and Pd: temperature 25-90oC- Nickel can also used as a catalyst, but a higher
temperature of 140oC – 200oC is needed.
Addition of halogens:
i) In inert solvent:- alkenes react with halogens at room temperature and in dark.- the halogens is usually dissolved in an inert solvent such as dichloromethane (CH2Cl2) and tetrachloromethane (CCl4).- Iodine will not react with alkenes because it is less reactive than chlorine and bromine.- Fluorine is very reactive. The reaction will produce explosion. C C X X C C
X X
inert solvent
X X = halogen such as Br2 or Cl2Inert solvent = CCl4 or CH2Cl2
EXAMPLES:
C CHH
H H Br Br
Br2
Br
Br
CCl4
CH3CH=CH2 Cl2CCl4 CH3CH
ClCH2
Cl
C CBr
H H
BrH H
inert solvent (CCl4)
ethene1,2-dibromoethane
* the red-brown colour of the bromine solution will fade and the solution becomes colourless.
cyclohexene 1,2-dibromocyclohexane
propene 1,2-dichloropropane
Hydrohalogenation
MARKOVNIKOV’S RULE ( A statement of the rule is not needed)
There are 2 possible products when hydrogen halides react with an unsymmetrical alkene.
It is because hydrogen halide molecule can add to the C=C bond in two different ways.
C C
H
HCH3
H
H-I
C C
H
HCH3
H
H-I
C C
H
HCH3
H
H I
C C
H
HCH3
H
I H
1-iodopropane
2-iodopropane(major product)
Markovnikov’s rules (Not for examination)
- the addition of HX to an unsymmetrical alkene, the hydrogen atom attaches itself to the carbon atom (of the double bond) with the larger number of hydrogen atoms.
Addition reaction with concentrated sulfuric acid: hydration of alkenes
- the alkene is absorbed slowly when it passed through concentrated sulfuric acid in the cold (0-15oC
Addition reaction with acidified water (H3O+): hydration of alkenes
• Hydration: The addition of H atoms and –OH groups from water molecules to a multiple bond.
• Reverse of the dehydration reaction.• Direct hydration of ethene: - passing a mixture of ethene and
steam over phosphoric (v) acid (H3PO4) absorbed on silica pellets at 300oC and a pressure of 60 atmospheres.
- H3PO4 is a catalyst.
CH2=CH2 H2OH3PO4 CH3CH2OH(g) (g)
300 oC, 60 atm(g)
ethene ethanol
C C H2O C CH OH
alkene alcohol
Oxidation (Combustion of alkenes)
The alkenes are highly flammable and burn readily in air, forming carbon dioxide and water.
For example, ethene burns as follows :
C2H4 + 3O2 → 2CO2 + 2H2O
HALOGENOALKANESHalogenoalkanes are compounds in which one
or more hydrogen atoms in an alkane have been replaced by halogen atoms (fluorine,
chlorine, bromine or iodine).
Functional group = halogen◦Ex. Fluorine = fluoro
Number by which carbon attached to, put in alphabetical order
Ex.
Bromoethane
Halogenoalkanes fall into different classes depending on how the halogen atom is positioned on the chain of carbon atoms. There are some chemical differences between the various types.
• Primary
• Secondary
• Tertiary
◦Primary (1°) – carbon carrying halogen is attached to only one carbon alkyl group
◦Secondary (2°)– carbon carrying halogen is attached to two other alkyl groups
◦Tertiary (3°) – carbon carrying halogen is attached to three alkyl groups
Reactions of the halogenoalkanes
Substitution:In a substitution reaction, one atom or group of atoms, takes the place of another in a molecule.
Elimination: Halogenoalkanes also undergo elimination reactions in the presence of sodium or potassium hydroxide which is
dissolved in ethanol.
Example of substitution
When an aqueous solution of NaOH or KOH is added to haloalkane an alcohol is produced.
propan-2-ol
Example of eliminationwhat conditions are needed?
ALCOHOLS (CnH2n+1OH)Preparation and properties
nomenclature
Select the longest chain which contains the OH group and number so that the OH group has the smallest number. See the examples below
Classification
In a primary (1°) alcohol, the carbon which carries the -OH group is only attached to one alkyl group.
In a secondary (2°) alcohol, the carbon with the -OH group attached is joined directly to two alkyl groups, which may be the same or different.
In a tertiary (3°) alcohol, the carbon atom holding the -OH group is attached directly to three alkyl groups, which may be any combination of same or different.
See the examples below
Alcohols are classified as primary, secondary or Tertiary
CH3 CH2 CH2 CH CH3
OH
CH3 CH2 CH2 C OH
CH3
CH3
CH3 CH2 CH2 CH2 OH
Reactions of alcohols
Alcohols contain an –OH group covalently bonded to a carbon atom.
We need know: the esterification reactionSubstitution and elimination
Preparation and reactions
1. By hydration of alkanesThe acid is absorbed in conc sulphuric
acid and then the acid is diluted.
2. Hydrolysis of halogenoalkanesThe halogen of the halogenoalkane is
replaced by an OH group Refer to Halogenoalkanes
Classic example for learners to write
CH3CHCH3
OHH2SO4
CH2 CHCH3H2
PtCH3CH2CH3
alcohol alkene alkane
esterification
Acid + Alcohol yields Ester + WaterSulfuric acid is a catalyst.Each step is reversible.
CH3 C OH
O
+ CH2CH2CHCH3
CH3
OHH+
CH3C
O
OCH2CH2CHCH3
CH3
+ HOH
=>
Chapter 11 72
Acid + Alcohol yields Ester + WaterSulfuric acid is a catalyst.Each step is reversible.
CH3 C OH
O
+ CH2CH2CHCH3
CH3
OHH+
CH3C
O
OCH2CH2CHCH3
CH3
+ HOH
=>
Aldehydes andKetones (Know the functional groups)
Nomenclature of Aldehydes: Select the longest carbon chain containing the carbonyl carbon. • The -e ending of the parent alkane name is
replaced by the suffix -al. • The carbonyl carbon is always numbered “1.” (It is not necessary to include the number in the name.) • Name the substituents attached to the chain in the usual way
Nomenclature of Ketones
No reactions. Just naming
SOME FUNCTIONAL GROUPS TO KNOW