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Chapter 2 Alkanes
Introduction1. Alkanes2. Alkyl Groups3. Nomenclature of Alkanes4. Isomerism in Alkanes5. Physical Properties of Alkanes6. Chemical Properties of Alkanes7. Preparation of Alkanes8. Methane9. Cycloalkanes
Table of Contents
Chapter 2
Warm up• What does “hydrocarbon” word imply?
• Where do you use hydrocarbons in life?
• List down common names of hydrocarbons you
commonly use.
Alkanes
Chapter 2 Introduction
Chapter 2 Introduction
• Hydrocarbons, as the name implies are compounds whose
molecules contain only carbon and hydrogen.
• They are extracted from petroleum, natural gas and coal.Hydrocarbons
Aliphatic Hydrocarbons Aromatic Hydrocarbons1. Saturated Hydrocarbons
Alkanes(Parafins)
2. Unsaturated Hydrocarbons
Alkenes(Olefins)
Alkynes(Acetylenes)
3. Cycloalkanes
(Benzene compounds)
Chapter 2 Introduction
Chapter 2 Introduction
Chapter 2 Introduction
Petroleum products
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.
H3CCH2
H2C
CH3
n-butane
H3CCH2
H2C
CH2
H2C
CH3
n-hexane• In branched alkanes, if one methyl,-CH3, group is on the second carbon the “iso-” prefix is added to name.
H3CCH
CH3
CH3iso-butane
H3CCH
H2C
CH3
iso-pentaneCH3
Chapter 2 1. Alkanes
• Additionally if there are two methyl,-CH3, groups are on the
second carbon the “neo-” prefix is added to name.
H3C C CH3
H3CC
H2C
CH3
CH3
CH3
neopentaneCH3
CH3
neohexane
Example 1What 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 4When 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.
• A carbon atom in an alkane can be named as primary,
secondary or tertiary according to number alkyl groups
bonded to it.
Chapter 2 2. Alkyl Groups
• CnH2n+2 → CnH2n+1
Alkane Alkyl
Example 5
C
H
H
H
H
methane
C
H
H H
methyl
CH4 CH3-
Chapter 2 2. Alkyl Groups
Example 6
C
H
R
H
primary
1oC
H
R
R'
secondary
2oC
R"
R
R'
tertiary
3o
C
H
H3C
primary
1oC
H
C2H5
CH3
secondary
2oC
CH3
C2H5
CH3
tertiary
3oH
Chapter 2
Example 7Name the carbon atoms shown in the following molecules as
primary, secondary or tertiary.
H3CCH
CH2
CH3
CH3
* * H3CC
CH2
CH3
CH3
**
CH31o3o
1o
2o
2. Alkyl Groups
Chapter 2 3. Nomenclature of Alkanes
• For simple alkanes common names are used.
H3C
H2C
CH2
CH3
H3CCH
CH2
CH3
CH3
n-butane isopentane
H3CC
CH3
CH3
Htertiary butane
• For the branched complex alkanes IUPAC system is used.
In the IUPAC system following rules can be used.
1. Determine the longest number of continuous carbon atom
chain. This gives the parent name of alkane.
Chapter 2 3. Nomenclature of Alkanes
H3CCH
CH2
CH six carbon in the longest chain refers to hexaneC
H2
CH3
CH3 CH3
2. Number the carbon atoms in the longest chain, starting
nearer at the end nearer to the branched.
H3CCH
CH2
CHCH2
CH3
CH3 CH3
12 3 4 65
Chapter 2 3. Nomenclature of Alkanes
3. 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)
H3CCH
CH2
CHCH2
CH3
CH3 CH3
2, 4-dimethylhexane
Chapter 2 3. Nomenclature of Alkanes
3. When two or more different groups are bonded to the
carbon chain, they are ordered alphabetically.
H3CCH
CHCH
CH2
CH3
CH3 Cl
4-chloro-3-ethyl-2-methylhexaneC2H5
Chapter 2 3. Nomenclature of Alkanes
Chapter 2
Example 8Name the following structures.
CH
H2CH3C
H2C CH3CH3C
HH3C
CH3a. b.
CH3
CH2CH3C
H2C CH2
CH3 CH3
C2H5
d.
CH2CH3C
H2C CH2
Br CH3
C3H7
e.
Isobutane or 2- methylpropane
3-methylpentane
3-ethyl-3-methylhexane 4-bromo-4-ethylheptane
3. Nomenclature of Alkanes
Chapter 2
Example 9Draw the structures of following alkanes.
3. Nomenclature of Alkanes
a. 3-ethyl-2-methylpentane
b. 1,3-dibromopropane
c. 2,2,3,3-tetramethylbutane
d. 2,2,4-trimethylpentane
e. 2-chloro-2,3-dimethyl-3-ethylheptane
f. 5-ethyl-2-methylheptane
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
H2C
H2CH3C CH3
CH3CH
H3C
CH3
n-butane 2-methylpropane (isobutane)
Example 10Write 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
R – H + X–X → R – X + H –X Halogenetion alkane halogen alkyl halide
Light
heat
CH4 + Cl2 → CH3Cl + HClhv
R – H + HO–NO2 → R–NO2 + H2O Nitration Alkane nitroalkane
425oC
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.
H3CCH2
H2C
CH2
H2C
CH3
H3CCH
H2C
CH2
CH3heat
AlCl3n-hexane
isohexane
CH3
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 12To 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 13What 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 11When 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.
R-MgXR-X + Mg Anhydrous
Ether Grignard Reagent
H-OHR-H + Mg(OH)X
Alkane
Example 15 Prepare ethane by using Grignard reagent.
CH3CH2MgBrCH3CH2Br + MgAnhydrous
Ether Ethyl Magnesium BromideGrignard Reagent
EthaneCH3CH2MgBr + H-OH CH3CH3 + Mg(OH)Br
Solution
Chapter 2 7. Preparation of Alkanes
3. Reduction of Alkyl Halides
Example 16 Prepare propane by using 2-bromopropane.
2R-H2R - XZn
H++ ZnX2
alkyl halide alkane
2CH3CH2CH3 + ZnBr22CH3CHCH3
Zn
H+Br
2-bromopropanepropane
Solution
Chapter 2 7. Preparation of Alkanes
4. Heating the Metal Salts of Carboxylic Acids
Example 17
Prepare methane by using sodium acetate.
Solution
HeatR-COONa + NaOH
AlkaneR-H + Na2CO3
sodium salt of a carboxylic acid
CaO
HeatCH3-COONa + NaOH
MethaneCH4 + Na2CO3
Sodium Acetate CaO
Chapter 2 7. Preparation of Alkanes
5. Hydrogenation of Unsaturated Hydrocarbons
Pt, Pd or Ni+ H2C C
alkene
C C
H Halkane
Pt, Pd or Ni+ 2H2C C
alkyneC C
H
H
H
H
alkane
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?
SolutionIn the mixture only C3H6 reacts with H2. C3H6 + H2 → C3H82 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
1500oCCH4 C2H2 + H2
1000oCCH4 C + 2H2
260oC 100 atm2CH4 + O2 2CH3OHcatalyst
Preparation
H+Al4C3 + 12H2O 4Al(OH)3 + 3CH4
Chapter 2 8. Methane
NiCO + 3H2
H2O + CH4300-400oC
NiC + 2H2
CH4pressure, heat
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.
H3C
H2C
CH3
propane
H2C
H2C CH2
cyclopropane cyclopropane
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.
H3C
H2C
CH2butane
H2C
H2C CH2
cyclobutane cyclobutane
CH3CH2
Methylcyclobutane
CH3Cl
Chlorocyclopentane
C2H5
Ethylcyclohexane
Chapter 2 9. Cycloalkanes
1-chloro-2-methylcyclobutane
CH3 C2H5
1-ethyl-3,4-dimethylcyclohexane
Cl1 2
H3C
H3C1
23
4
• If the number of carbon atom in the ring is less than that
is in the substituent, then they are named as
cycloalkylalkanes.
H3C
H2C
CH
3-cyclopropylhexane
H2C
CH2
CH3H2C
CH2
H2C
CH2
H3C
1-cyclobutylpentane
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
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