Bài 25. Ankan

Chapter 2
Alkanes
Introduction
Alkanes
Alkyl Groups
Nomenclature of Alkanes
Isomerism in Alkanes
Physical Properties of Alkanes
Chemical Properties of Alkanes
Preparation of Alkanes
Methane
Cycloalkanes
Table of Contents
Chapter 2
Introduction
Chapter 2
HYDROCARBONS
Hydrocarbons, as the name implies are compounds whose molecules contain only carbon and hydrogen.
They are extracted from petroleum, natural gas and coal.
Chapter 2
Introduction
Chapter 2
Introduction
Chapter 2
1. Alkanes
Contain C-C and C-H sigma bonds.
Known as paraffin, meaning inert.
General formula is CnH2n+2, n is number of carbon atoms.
First four members have common names, methane, ethane, propane and butane.
Names end with –ane suffix.
In the names “latin number + -ane ” is used, for example penta + ane = pentane
-CH2-, methylene, is difference between consecutive alkanes, forming homologous series.
Chapter 2
1. Alkanes
Chapter 2
1. Alkanes
First 10 Members of Alkanes
Chapter 2
1. Alkanes
Straight chain alkanes take n- in front of name, meaning “normal” and shows the alkane is unbranched.
In branched alkanes, if one methyl,-CH3, group is on the second carbon the “iso-” prefix is added to name.
Chapter 2
1. Alkanes
Additionally if there are two methyl,-CH3, groups are on the second carbon the “neo-” prefix is added to name.
Example 1
What is the molecular formula of the alkane having the 58 g/mole molecular mass?
Chapter 2
1. Alkanes
Example 2
14.4 g of an alkane, when analyzed, is found to contain 12 g carbon. What is its molecular formula?
Example 3
If a 3 g sample of hydrocarbon is burned, 5.4 g of water vapor is produced. What is the molecular formula of hydrocarbon?
Example 4
When the CO2 gas produced by the combustion 0.2 mol of an alkane is passed into limewater (Ca(OH)2), 40 g of calcium carbonate precipitates. What is the molecular formula of the alkane?
Chapter 2
2. Alkyl Groups
If one hydrogen atom is removed from an alkane, an alkyl group is formed.
General formula of alkyl groups is CnH2n+1.
Instead of the “–ane” suffix in alkanes, “-yl” is used for naming of alkyl groups.
They are represented as “R”, meaning radical.
Chapter 2
2. Alkyl Groups
CnH2n+2 → CnH2n+1
Alkane Alkyl
Example 5
Chapter 2
2. Alkyl Groups
Example 6
Chapter 2
Example 7
Name the carbon atoms shown in the following molecules as primary, secondary or tertiary.
1o
3o
1o
2o
2. Alkyl Groups
Chapter 2
3. Nomenclature of Alkanes
For simple alkanes common names are used.
For the branched complex alkanes IUPAC system is used. In the IUPAC system following rules can be used.
Determine the longest number of continuous carbon atom chain. This gives the parent name of alkane.
Chapter 2
3. Nomenclature of Alkanes
Number the carbon atoms in the longest chain, starting
nearer at the end nearer to the branched.
Chapter 2
3. Nomenclature of Alkanes
Show the position of groups bonded to parent chain, if there are more than one identical groups, use –di, -tri or -tetra prefixes. Some important groups are –F (fluoro), -Cl (chloro), -Br (bromo), –I (iodo), -NO2(nitro), -OH (hydroxyl), and –NH2(amino)
Chapter 2
3. Nomenclature of Alkanes
When two or more different groups are bonded to the carbon chain, they are ordered alphabetically.
Chapter 2
3. Nomenclature of Alkanes
Chapter 2
Example 8
Name the following structures.
Isobutane or
2- methylpropane
3-methylpentane
3-ethyl-3-methylhexane
4-bromo-4-ethylheptane
3. Nomenclature of Alkanes
Chapter 2
Example 9
Draw the structures of following alkanes.
3. Nomenclature of Alkanes
3-ethyl-2-methylpentane
1,3-dibromopropane
2,2,3,3-tetramethylbutane
2,2,4-trimethylpentane
2-chloro-2,3-dimethyl-3-ethylheptane
5-ethyl-2-methylheptane
Name these molcules
3-ethyl-3,4-dimethylhexane
Chapter 2
Compounds having same molecular formula but different structural formula are said to be isomers.
First three members of alkanes do not have isomers.
Physical and chemical properties of isomers are different.
4. Isomerism in Alkanes
Example 10
Write the structural formulae and the IUPAC name of possible isomers of hexane, C6H14.
Chapter 2
4. Isomerism in Alkanes
Chapter 2
Alkanes are non polar molecules, only van der waals forces are responsible for the attractions between molecules.
C1-C4 are gases, C5-C17 are liquids, and the rest are solids.
Bp, Mp, and densities of alkanes increase by the increasing number of carbon atom.
They are generally tasteless, odorless and colorless.
Branching decreases boiling point.
They are insoluble in water, but soluble in organic solvents such as benzene and carbon tetrachloride.
5. Physical Properties of Alkanes
Chapter 2
Another name for alkanes is paraffin, meaning inert. They are chemically quite unreactive.
They do not react with dilute acids (HCl, H2SO4), active metals(Na, K) and strong oxidizing agents(KMnO4, Na2Cr2O4).
They only undergo combustion and substitution reaction with halogens. They can be nitrated under high pressure.
6. Chemical Properties of Alkanes
1. Combustion Reaction
CnH2n+2 + (3n+1)/2 O2 → n CO2 + (n+1) H2O
CH4 + 2O2 → CO2 + 2H2O
Chapter 2
6. Chemical Properties of Alkanes
Chapter 2
6. Chemical Properties of Alkanes
2. Substitution Reaction
Chapter 2
6. Chemical Properties of Alkanes
2. Substitution Reaction
Chapter 2
6. Chemical Properties of Alkanes
4. Isomerization Reaction
Straight-chain alkanes are converted into branched alkanes.
3. Cracking Reactions
Higher alkanes are converted into smaller alkanes and alkenes at high temperature. When a catalyst is used in the process, it is known as catalytic cracking.
Chapter 2
6. Chemical Properties of Alkanes
Example 12
To burn a mixture of 33.6 L of methane and propane gas at STP, 144 g of oxygen gas are required. According to this information, what is the total mass of the original mixture? (H:1, C:12, O:16 )
Example 13
What is the pressure of CO2 gas in 2-L container which is produced from the combustion of 7.2 g of C5H12 at 27oC?
Example 11
When 8.8 g of an alkane is burned completely, 26.4 g of carbon dioxide and 14.4 g of water are produced. What is the molecular formula of the alkane?
Chapter 2
Main sources of alkanes are crude oil, natural gas and coal. Many useful alkanes are obtained from fractional distillation of petroleum.
7. Preparation of Alkanes
1. Wurtz Synthesis
This method was first used by Adolph Wurtz in 1854.
Methane cannot be produced in this process.
R-X + 2Na + X-R → R-R + 2NaX

2CH3-Br + 2Na → CH3-CH3 + 2NaBr
Chapter 2
Example 14
7. Preparation of Alkanes
Which alkyl halides can be used to prepare pentane by the Wurtz synthesis.
Example 15
What is(are) the products of the reaction of 2-chloropropane and 1-bromoethane with excess sodium metal?
Chapter 2
7. Preparation of Alkanes
2. Hydration of Grignard Compounds
This method was first used by Victor Grignard in 1912.
Example 15
Prepare ethane by using Grignard reagent.
Solution
Chapter 2
7. Preparation of Alkanes
3. Reduction of Alkyl Halides
Example 16
Prepare propane by using 2-bromopropane.
Solution
Chapter 2
7. Preparation of Alkanes
4. Heating the Metal Salts of Carboxylic Acids
Example 17
Prepare methane by using sodium acetate.
Solution
Chapter 2
7. Preparation of Alkanes
5. Hydrogenation of Unsaturated Hydrocarbons
Chapter 2
7. Preparation of Alkanes
Example 17
5 mol mixture of C3H6 and C3H8 is reacted with 2 mol of H2 gas. What is the total volume of gases at STP when the reaction is completed?
Solution
In the mixture only C3H6 reacts with H2.
C3H6 + H2 → C3H8
2 mol H2 → 2 mol C3H6, the mol of C3H8 = 5 – 2 = 3 mol is unreacted.
nT = 2 mol C3H8 produced + 3 mol C3H8 unreacted = 5 mol
V= 5x22.4 = 112 L
Ni
Chapter 2
8. Methane
It is found in mines and in natural gas.
It is also known as marsh gas and is formed by decomposition of plants in the absence of oxygen.
It is colorless and odorless has.
It is insoluble in water, but soluble in benzene and gasoline.
10-15% of methane in air may cause an explosion. It is called in mines “firedamp explosions”.
It burns with a light blue flame.
Chapter 2
8. Methane
Preparation
Chapter 2
8. Methane
Example 18
What is the percent of a 150 g of Al4C3 sample that produces 67.2 L of CH4 gas when put in water? (Al:27, H:1, C:12)
Chapter 2
9. Cycloalkanes
Alkanes bonded to one another in a ring formation is called cycloalkanes.
General formula is CnH2n.
Nomenclature of Cycloalkanes
Use cyclo- prefix to the names of alkanes.
Chapter 2
9. Cycloalkanes
If there are more than one substituent in the ring, the ring is numbered starting with the substituent first in the alphabet, and numbered in the direction which gives the lowest number for the second substituent.
Chapter 2
9. Cycloalkanes
If the number of carbon atom in the ring is less than that is in the substituent, then they are named as cycloalkylalkanes.
Chapter 2
9. Cycloalkanes
Chapter 2
9. Cycloalkanes
Melting point, boiling point and densities of cycloalkanes are different from the alkanes possessing the same number of alkanes.
Melting and boiling points increase as the number of carbon atom increases in the ring.
Physical Properties
Chemical Properties
Cycloalkanes with more than 5 carbon atom undergo substitution reaction.
Cycloalkanes with 3 or 4 carbon atoms undergo addition reaction.
Chapter 2
9. Cycloalkanes
End of the chapter 2