13 alkanes

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Chapter 13 Introduction to Organic Chemistry: Alkanes

13 Alkanes

Copyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings

STRUCTURES OF ALKANES

METHANE CH4

Bond Angle 109.5o Shape Tetrahedral

Can be illustrated as:

H

H C H

H

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Structural Formulas

Alkanes are written with structural formulas that are• expanded to show each bond.• condensed to show each carbon atom and its

attached hydrogen atoms.

Expanded Condensed H

H C H CH4 , methane

H

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Expanded and Condensed Structures

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Structural Formulas

Condensed formulas are written for expanded structural formula by showing each carbon and the attached hydrogen atoms.

Expanded Condensed H H H H │ │ │ │H─C ─C ─C ─C ─ H CH3─CH2─CH2─CH3

│ │ │ │ H H H H

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Names of Alkanes

The names of alkanes • are determined by the IUPAC (International Union of

Pure and Applied Chemistry) system.• end in –ane.• with 1-4 carbons in a chain use prefixes as follows.

Name # Carbons Structural Formula

Methane1 CH4

Ethane 2 CH3CH3

Propane 3 CH3CH2CH3

Butane 4 CH3CH2CH2CH3

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Names of Alkanes

Alkanes with 5-10 carbon atoms in a chain use Greek prefixes.Name # Carbons Structural Formula

Pentane 5 CH3CH2CH2CH2CH3

Hexane 6 CH3CH2CH2CH2CH2CH3

Heptane 7 CH3CH2CH2CH2CH2CH2CH3

Octane 8 CH3CH2CH2CH2CH2CH2CH2CH3

Nonane 9 CH3 CH2 CH2CH2CH2CH2CH2CH2CH3

Decane 10 CH3CH2CH2CH2CH2CH2CH2CH2CH2CH3

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Learning Check

A. Write the condensed formula for: H H H H H

H C C C C C H

H H H H H

B. What is its molecular formula?C. What is its name?

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Solution

A. CH3─CH2─CH2─CH2─CH3

B. C5H12

C. pentane

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Writing Structural Formulas

Carbon atoms in a chain

• maintain tetrahedral shape.• are connected in a zig-zag pattern.• are drawn as 2-dimensional. • Ccan be written in several conformations.

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Some Structures for Butane

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Hexane has Six Carbon Atoms

Hexane • is an alkane with six carbon atoms in a continuous

chain.• has a “zig-zag” look because each carbon atom is at

the center of a tetrahedron.• is represented by a ball-and-stick model as shown

below.

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Learning Check

Write the condensed structural formula for

A. ethane.

B. heptane.

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Solution

Write the condensed structural formula for

A. ethane CH3─CH3

B. heptane CH3─CH2─CH2─CH2─CH2─CH2─CH3

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Cycloalkanes

Cycloalkanes

• are cyclic alkanes.

• have two hydrogen atoms fewer than the open chain.

• are named by using the prefix cyclo- before the name of the alkane chain with the same number of carbon atoms.

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Cycloalkanes

The structural formulas of cycloalkanes are usually represented by geometric figures,

Cyclopropane CH2

CH2 CH2

CyclobutaneCH2 CH2

CH2 CH2

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More Cycloalkanes

Cyclopentane CH2

CH2 CH2

CH2 CH2

Cyclohexane CH2

CH2 CH2

CH2 CH2

CH2

CH2

CH2

CH2CH2

CH2

CH2

CH2

CH2

CH2

CH2

CH2

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Learning Check

Name the following.

A. CH3─CH2─CH2─CH3

B.

C. CH3─CH2─CH2─CH2─CH2─CH2─CH2─CH3

D.

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Solution

Name the following:

A. CH3─CH2─CH2─CH3 butane

B. cyclopropane

C. CH3─CH2─CH2─CH2─CH2─CH2─CH2─CH3 octane

D. cyclohexane

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Types of alkanes

Alkanes

branched

Iso alkanes

Neo alkanes

straight

The alkanes in which one methyl group is attached to second carbon atom of normal chain of carbon atom is called isoalkanes

The alkanes in which two methyl group is attached to second carbon atom of normal chain of carbon atom is called neoalkanes

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Types of carbon atoms

Type of C atom

primary secondary Tertiary Quaternary

CH2

CH3

CH2

CH3

CH3

CH3

CH3C

CH

“classes of carbons”

primary carbon (1o) – a carbon bonded to one carbon

secondary carbon (2o) – a carbon bonded to two carbons

tertiary carbon (3o) – a carbon bonded to three carbons

quaternary carbon (4o) – a carbon bonded to four carbons

1o

4o

CH3 CH3CH3CHCH2CH2CCH3 1o

CH3 3o 2o

classification of hydrogens, halides – hydrogens or halides are classified by the carbon to which they are attached.

1o

CH3CH3CHCH2CH2CH3

1o 3o 2o 2o 1o

CH3CH2CHCH3 sec-butyl bromide 2o bromide Br

CH3CH3CCH3 tert-butyl chloride 3o chloride Cl

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Nomenclature

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Isomers of Butane

Isomers• have the same

molecular formula.• have different atom

arrangements.• of butane (C4H10) are a

straight chain and a branched chain.

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Alkyl groups

Alkyl groups are• alkanes that are missing one H. • substituents attached to carbon chains.• named with a –yl ending.

H

H C CH3 methyl

H H H H C C CH3 CH2 ethyl H H

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Naming Substituents

In the IUPAC system,• a carbon branch is

named as an alkyl group.

• halogen atoms are named as halo.

TABLE 10.6

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Alkanes with Substituents

CH3

CH3 CH CH3 methylpropane

methyl groups

CH3 CH3

CH3 CH CH2 CH CH3 2,4-dimethylpentane

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Naming Alkanes

Give the name of CH3 CH3

CH3─CH─CH─CH3

STEP 1 Name the longest continuous chain. CH3 CH3

CH3─CH─CH─CH3

butane

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Naming Alkanes

Give the name of CH3 CH3

CH3─CH─CH─CH3

STEP 2 Number chain from end nearest substituent .

CH3 CH3

CH3─CH─CH─CH3

1 2 3 4

STEP 3 Locate substituents and name alphabetically. 2,3-dimethylbutane

Note: when more substituents present on same carbon atom, its gets priority

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Learning Check

Write the name of

Cl CH3

CH3─CH2─CH─CH─CH3

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Solution

STEP 1 Longest chain is pentane.

STEP 2 Number chain from end nearest substituent. Cl CH3

CH3─CH2─CH─CH─CH3

5 4 3 2 1

STEP 3 Locate substituents and name alphabetically. 3-chloro-2-methylpentane

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Learning Check

Give the IUPAC name for each of the following:

A. CH3 CH3

| |

CH3─CH─CH2 ─CH─CH3

B. Cl CH3 | |

CH3─CH2─CH─CH2─C─CH2─CH3

|

Cl

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Solution

A. CH3 CH3

| | CH3─CH─CH2 ─CH─CH3 2,4-dimethylpentane

1 2 3 4 5

B. Cl CH3 more substituents = C3 | |

CH3─CH2─CH─CH2─C─CH2─CH3

|

Cl 7 6 5 4 3 2 1

3,5-dichloro-3-methylheptane

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Learning Check

Draw the condensed structural formula for 2-bromo-4-chlorobutane.

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Solution

2-bromo-4-chlorobutaneSTEP 1 Longest chain has 4 carbon atoms.

C─C─C─CSTEP 2 Number chain and add substituents.

Br

C ─ C ─ C ─ C ─ Cl 1 2 3 4

STEP 3 Add hydrogen to complete 4 bonds to each C. Br

CH3─CH─CH2─CH2─Cl

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Naming Cycloalkanes with Substituents

The name of a substituent is placed in front of thecycloalkane name.

methylcyclobutane CH3

chlorocyclopentane Cl

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Learning Check

Name each of the following.

1. CH3

CH2─CH3

2.

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Solution

Name each of the following.

1. methylcyclopropane

2. ethylcyclohexane

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Properties

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Conformation of alkanes

ETHANE. Molecular formula C2H6

Structural formula: CH3 CH3 or H H

H C C H

H H

PROPANE.

Molecular formula: C3H8

Structural formula: CH3 CH2 CH3 or H H H

H C C C H

H H HBoth ethane and propane are “straight” chain molecules

BUT!! Bonds are NOT 90o molecules are NOT STRAIGHT!!!

Schematic formula

Molecular formula: C4H10 - can have two different structures

“Straight” chain. CH3 CH2 CH2 CH3 BUTANE

Branched chain CH3 CH CH3

CH3

METHYL PROPANE

Isomers

Compounds that have the same molecular formula but different structural formula.

branch

BUTANE & ISOMERS.

Schematic formula:

Names & Structures of three different isomers C5H12.

Examples

2- methylbutane

CH3

CH3 CH CH2 CH3

CH3

CH3 C CH3

CH3

2,2 – dimethyl propane

TASK: illustrate the structures of:

2-methylpentane.

2,3 – dimethylbutane.

2,2,3 -trimethylpentane

CH3CH(CH3)CH2CH2CH3

CH3CH(CH3)CH(CH3)CH3

CH3C(CH3)2CH(CH3)CH2CH3

THE RULES FOR NAMING ORGANIC COMPOUNDS

1. Choose the longest unbroken chain of Carbon atoms and assign a name for the carbon chain using the prefixes; meth-1, eth-2 etc.

2. Identify any carbon chain branches (alkyl groups). These are assigned names using the same prefixes as above along with the suffix “-yl” – methyl, ethyl etc.

3. Identify the functional groups present in the molecule. Assign a prefix or suffix according to their homologous series. These will be written in front of the name of the carbon chain.

4. There is an order of precedence, to decide the suffix for the carbon chain:

COOH / C=C > OH > Br / Cl

5. Number the Carbon atoms in the longest chain so that the branches/functional groups have the lowest number possible. Allocate a number for every group/branch no matter how many times it occurs. Where groups are on the same carbon write their names in alphabetical order.

6. Numbering takes precedence "wins" over alphabetical spelling. Prefixes are used for groups that occur more than once.

Di – 2 Tri – 3 Tetra – 4 Penta – 5 etc.

7. The final name is written as one word with commas between numbers, hyphens separating numbers from words.

Give the names of the following alkanes

3-methyl pentane

2,4-dimethylpentane

2,2,4-trimethyl

hexane

(a) CH3 CH2 CH CH2 CH3

CH3

(b) CH3 CH CH2 CH CH3

CH3 CH3

(c) CH3 C(CH3)2 CH2 CH(CH3) CH2 CH3

(d) CH3CH2CH(CH3)C(CH3)3 2,2,3-trimethylpentane

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Chapter 13 Introduction to Organic Chemistry: Alkanes

13 Properties of Alkanes

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Some Properties of Alkanes

Alkanes are• nonpolar.• insoluble in water.• less dense than water.• flammable in air.

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Some Properties of Alkanes

Alkanes with 1-4 carbon atoms are

• methane, ethane, propane, and butane.

• gases at room temperature.

• used as heating fuels.

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Some Properties of Alkanes

Alkanes with 5-8 carbon atoms are• liquids at room temperature.• pentane, hexane, heptane, and octane.• very volatile.• used to make gasoline.

Alkanes with 9-17 carbon atoms • are liquids at room temperature• have higher boiling points.• are found in kerosene, diesel, and jet fuels.

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Some Properties of Alkanes

Alkanes with 18 or more carbon atoms• have high molar masses.• are waxy solids at room temperature.• used in waxy coatings of fruits and

vegetables.

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Melting and BoilingPoints of Hydrocarbons

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Combustion

In combustion reactions,• alkanes react with oxygen. • CO2, H2O and energy are produced.

Alkane + O2 CO2 + H2O + heat

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Learning Check

Write a balanced equation for thecomplete combustion of propane.

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Solution

Unbalanced equation

C3H8 + O2 CO2 + H2O

Balance C

C3H8 + O2 3CO2 + H2O

Balance H

C3H8 + O2 3CO2 + 4H2O

Balance O

C3H8 + 5O2 3CO2 + 4H2O (Balanced)

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CH4 + Cl2 → CH3Cl + HCl

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