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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


• 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)


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.


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


• 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?


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.


• CnH2n+2 → CnH2n+1

Alkane Alkyl

Example 5

C

H

H

H

H

methane

C

H

H H

methyl

CH4 CH3-


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.


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


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


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

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


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


2. Substitution Reaction


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.


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


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


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


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


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.


1500oCCH4 C2H2 + H2

1000oCCH4 C + 2H2

260oC 100 atm2CH4 + O2 2CH3OHcatalyst

Preparation

H+Al4C3 + 12H2O 4Al(OH)3 + 3CH4


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


• 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


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


• 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.

End of the chapter 2

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