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ORGANIC CHEMISTRY 171. Section 201. Alkanes and Cycloalkanes. Chapter 2. Structure. Hydrocarbon: a compound composed only of carbon and hydrogen Saturated hydrocarbon: a hydrocarbon containing only single bonds

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Organic chemistry 171


Section 201

Organic chemistry 171




Chapter 2



  • Hydrocarbon: a compound composed only of carbon and hydrogen

  • Saturated hydrocarbon:a hydrocarbon containing only single bonds

  • Alkane: a saturated hydrocarbon whose carbons are arranged in an open chain

  • Aliphatic hydrocarbon:another name for an alkane



Functional groups

Functional Groups

  • Functional group: An atom or group of atoms within a molecule that shows a characteristic set of physical and chemical properties.

  • Functional groups are important for three reasons:

    1. Allow us to divide compounds into classes.

    2. Each group undergoes characteristic chemical reactions.

    3. Provide the basis for naming compounds.

Organic chemistry 171

Organic Molecules and Functional Groups

Functional Groups: Hydrocarbons

Hydrocarbons are compounds made up of only the elements carbon and hydrogen. They may be aliphatic or aromatic.

Organic chemistry 171

Organic Molecules and Functional Groups

Functional Groups: Heteroatoms

Organic chemistry 171

Functional Groups: Carbonyl groups



  • Contain an -OH (hydroxyl) group bonded to a tetrahedral carbon atom.

  • Ethanol may also be written as a condensed structural formula.



  • Alcohols are classified as primary (1°), secondary (2°), or tertiary (3°) depending on the number of carbon atoms bonded to the carbon bearing the -OH group.



There are two alcohols with molecular formula C3H8O



  • Contain an amino group; an sp3-hybridized nitrogen bonded to one, two, or three carbon atoms.

    • An amine may by 1°, 2°, or 3°.

Aldehydes and ketones

Aldehydes and Ketones

  • Contain a carbonyl (C=O) group.

Carboxylic acids

Carboxylic Acids

  • Contain a carboxyl (-COOH) group.

Carboxylic esters

Carboxylic Esters

  • Ester: A derivative of a carboxylic acid in which the carboxyl hydrogen is replaced by a carbon group.

Carboxylic amide

Carboxylic Amide

  • Carboxylic amide, commonly referred to as an amide: A derivative of a carboxylic acid in which the -OH of the -COOH group is replaced by an amine.

    • The six atoms of the amide functional group lie in a plane with bond angles of approximately 120°.

Organic chemistry 171




  • Shape

    • tetrahedral about carbon

    • all bond angles are approximately 109.5°

    • sp3 hybridization

Drawing alkanes

Drawing Alkanes

  • Line-angle formulas

    • an abbreviated way to draw structural formulas

    • each vertex and line ending represents a carbon

Constitutional isomerism

Constitutional Isomerism

  • Constitutional isomers: compounds with the same molecular formula but a different connectivity of their atoms

    • example: C4H10

Constitutional isomerism1

Constitutional Isomerism

  • do these formulas represent constitutional isomers?

  • find the longest carbon chain

  • number each chain from the end nearest the first branch

  • compare chain lengths as well the identity and location of branches

Constitutional isomerism2

Constitutional Isomerism

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Nomenclature iupac

Nomenclature - IUPAC

  • Suffix -ane specifies an alkane

  • Prefix tells the number of carbon atoms

Nomenclature iupac1

Nomenclature - IUPAC

  • Parent name: the longest carbon chain

  • Substituent: a group bonded to the parent chain

    • alkyl group: a substituent derived by removal of a hydrogen from an alkane; given the symbol R-

Nomenclature iupac2

Nomenclature - IUPAC

1.The name of a saturated hydrocarbon with an unbranched chain consists of a prefix and suffix

2. The parent chain is the longest chain of carbon atoms

3. Each substituent is given a name and a number

4. If there is one substituent, number the chain from the end that gives it the lower number

Nomenclature iupac3

Nomenclature - IUPAC

5. If there are two or more identical substituents, number the chain from the end that gives the lower number to the substituent encountered first

  • indicate the number of times the substituent appears by a prefix di-, tri-, tetra-, etc.

  • use commas to separate position numbers

Nomenclature iupac4

Nomenclature - IUPAC

6. If there are two or more different substituents,

  • list them in alphabetical order

  • number from the end of the chain that gives the substituent encountered first the lower number

Nomenclature iupac5

Nomenclature - IUPAC

7. The prefixes di-, tri-, tetra-, etc. are not included in alphabetization

  • alphabetize the names of substituents first and then insert these prefixes

Nomenclature iupac6

Nomenclature - IUPAC

  • Alkyl groups

Nomenclature common

Nomenclature - Common

  • The number of carbons in the alkane determines the name

    • all alkanes with four carbons are butanes, those with five carbons are pentanes, etc.

    • iso- indicates the chain terminates in -CH(CH3)2; neo- that it terminates in -C(CH3)3

Classification of c h

Classification of C & H

  • Primary (1°) C: a carbon bonded to one other carbon

    • 1° H: a hydrogen bonded to a 1° carbon

  • Secondary (2°) C: a carbon bonded to two other carbons

    • 2° H: a hydrogen bonded to a 2° carbon

  • Tertiary (3°) C: a carbon bonded to three other carbons

    • 3° H: a hydrogen bonded to a 3° carbon

  • Quaternary (4°) C: a carbon bonded to four other carbons



  • General formula CnH2n

    • five- and six-membered rings are the most common

  • Structure and nomenclature

    • to name, prefix the name of the corresponding open-chain alkane with cyclo-, and name each substituent on the ring

    • if only one substituent, no need to give it a number

    • if two substituents, number from the substituent of lower alphabetical order

    • if three or more substituents, number to give them the lowest set of numbers and then list substituents in alphabetical order



  • Line-angle drawings

    • each line represents a C-C bond

    • each vertex and line ending represents a C



  • Example: name these cycloalkanes

Iupac general

Nature of Carbon-Carbon

Bonds in the Parent Chain

IUPAC - General

  • prefix-infix-suffix

    • prefix tells the number of carbon atoms in the parent

    • infix tells the nature of the carbon-carbon bonds

    • suffix tells the class of compound







all single bonds




one or more double bonds




one or more triple bonds





-oic acid

carboxylic acid

Iupac general1

IUPAC - General

prop-en-e = propene

eth-an-ol = ethanol

but-an-one = butanone

but-an-al = butanal

pent-an-oic acid = pentanoic acid

cyclohex-an-ol = cyclohexanol

eth-yn-e = ethyne

eth-an-amine = ethanamine



  • Conformation: any three-dimensional arrangement of atoms in a molecule that results from rotation about a single bond

  • Newman projection: a way to view a molecule by looking along a carbon-carbon single bond



  • Staggered conformation:a conformation about a carbon-carbon single bond in which the atoms or groups on one carbon are as far apart as possible from the atoms or groups on an adjacent carbon



  • Eclipsed conformation: a conformation about a carbon-carbon single bond in which the atoms or groups of atoms on one carbon are as close as possible to the atoms or groups of atoms on an adjacent carbon



  • Torsional strain

    • also called eclipsed interaction strain

    • strain that arises when nonbonded atoms separated by three bonds are forced from a staggered conformation to an eclipsed conformation

    • the torsional strain between eclipsed and staggered ethane is approximately 12.6 kJ (3.0 kcal)/mol



  • Dihedral angle (Q): the angle created by two intersecting planes



  • Steric strain(nonbonded interaction strain):

    • the strain that arises when atoms separated by four or more bonds are forced closer to each other than their atomic (contact) radii will allow

  • Angle strain:

    • strain that arises when a bond angle is either compressed or expanded compared to its optimal value



  • angle strain: the C-C-C bond angles are compressed from 109.5° to 60°

  • torsional strain: there are 6 sets of eclipsed hydrogen interactions

  • strain energy is about 116 kJ (27.7 kcal)/mol



  • Chair conformation: the most stable puckered conformation of a cyclohexane ring

    • all bond C-C-C bond angles are 110.9°

    • all bonds on adjacent carbons are staggered



  • In a chair conformation, six H are equatorial and six are axial



  • For cyclohexane, there are two equivalent chair conformations

    • all C-H bonds equatorial in one chair are axial in the alternative chair, and vice versa



  • Boat conformation: a puckered conformation of a cyclohexane ring in which carbons 1 and 4 are bent toward each other

    • there are four sets of eclipsed C-H interactions and one flagpole interaction

    • a boat conformation is less stable than a chair conformation by 27 kJ (6.5 kcal)/mol



  • Twist-boat conformation

    • approximately 41.8 kJ (5.5 kcal)/mol less stable than a chair conformation

    • approximately 6.3 kJ (1.5 kcal)/mol more stable than a boat conformation



  • Equatorial and axial methyl conformations

Cis trans isomerism

Cis,Trans Isomerism

  • Stereoisomers: compounds that have

    • the same molecular formula

    • the same connectivity

    • a different orientation of their atoms in space

  • Cis,transisomers

    • stereoisomers that are the result of the presence of either a ring (this chapter) or a carbon-carbon double bond (Chapter 5)

Cis trans isomers

Cis,Trans Isomers

  • 1,2-Dimethylcyclopentane

Cis trans isomerism1

Cis,Trans Isomerism

  • 1,4-Dimethylcyclohexane

Cis trans isomerism2

Cis,Trans Isomerism

  • trans-1,4-Dimethylcyclohexane

    • the diequatorial-methyl chair conformation is more stable by approximately 2 x (7.28) = 14.56 kJ/mol

Cis trans isomerism3

Cis,Trans Isomerism

  • cis-1,4-Dimethylcyclohexane

Physical properties

Physical Properties

  • Low-molecular-weight alkanes (methane....butane) are gases at room temperature

  • Higher molecular-weight alkanes (pentane, decane, gasoline, kerosene) are liquids at room temperature

  • High-molecular-weight alkanes (paraffin wax) are semisolids or solids at room temperature

Physical properties1

Physical Properties

  • Constitutional isomers have different physical properties

Preparation and reactions of alkanes

Preparation and Reactions of Alkanes

Preparation of alkanes via reduction

Preparation of Alkanes via Reduction

1) Hydrogenation of Alkenes

Hydrogenation of Alkynes

Organic chemistry 171

  • Reduction of an alkyl halide

  • a) hydrolysis of a Grignard reagent (two steps)

  • i) R—X + Mg  RMgX (Grignard reagent)

  • ii) RMgX + H2O  RH + Mg(OH)X


  • CH3CH2CH2-Br + Mg  CH3CH2CH2-MgBr

  • n-propyl bromiden-propyl magnesium bromide

  • CH3CH2CH2-MgBr + H2O  CH3CH2CH3 + Mg(OH)Br

  • propane

Organic chemistry 171


CH3CH-Br + Mg  CH3CH-MgBr

isopropyl bromide isopropyl magnesium bromide


CH3CH-MgBr + H2O  CH3CH2CH3



heavy water



Organic chemistry 171

  • with an active metal and an acid

  • R—X + metal/acid  RH

  • active metals = Sn, Zn, Fe, etc.

  • acid = HCl, etc. (H+)

  • CH3CH2CHCH3 + Sn/HCl  CH3CH2CH2CH3 +

  • Cl

  • sec-butyl chloriden-butane

  • CH3 CH3

  • CH3CCH3 + Zn/H+  CH3CHCH3 + ZnBr2

  • Br

  • tert-butyl bromideisobutane


Organic chemistry 171

Reactions of alkanes:

alkane + H2SO4 no reaction (NR)

alkane + NaOH  NR

alkane + Na  NR

alkane + KMnO4  NR

alkane + H2,Ni  NR

alkane + Br2  NR

alkane + H2O  NR

(Alkanes are typically non-reactive. They don’t react with acids, bases, active metals, oxidizing agents, reducing agents, halogens, etc.)

Organic chemistry 171

  • Alkane, reactions:

  • Halogenation

  • 2. Combustion (oxidation)

  • 3. Pyrolysis (cracking)

Organic chemistry 171

  • Halogenation

  • R-H + X2, heat or hv  R-X + HX

  • a) heat or light required for reaction.

  • b) X2: Cl2 > Br2 I2

  • c) yields mixtures 

  • d) H: 3o > 2o > 1o > CH4

  • e) bromine is more selective

Organic chemistry 171

CH3CH3 + Cl2, hv  CH3CH2-Cl + HCl

ethane ethyl chloride

CH3CH2CH3 + Cl2, hv  CH3CH2CH2-Cl + CH3CHCH3

propanen-propyl chloride Cl

isopropyl chloride 45%


gives a mixture of both the possible

alkyl halides! 

Organic chemistry 171

CH3CH2CH2CH3 + Cl2, hv  CH3CH2CH2CH2-Cl

n-butanen-butyl chloride 28%




sec-butyl chloride


CH3CHCH3 + Cl2, hv  CH3CHCH2-Cl 64%

isobutane isobutyl chloride



CH3CCH3 36%


tert-butyl chloride

Organic chemistry 171

  • chlorination of propane, mechanism:

  • abstraction of 1o hydrogen:

  • Cl• + CH3CH2CH3 CH3CH2CH2• + HCl

  • or abstraction of 2o hydrogen:

  • Cl• + CH3CH2CH3  CH3CHCH3 + HCl

  • 2)CH3CH2CH2• + Cl2  CH3CH2CH2Cl + Cl•

  • or CH3CHCH3 + Cl2  CH3CHCH3 + Cl•

  • • Cl

    • plus terminating steps 3) Cl—Cl  2 Cl•

2 oxidation of alkanes combustion

2.Oxidation of Alkanes ( combustion )

  • Oxidation is the basis for their use as energy sources for heat and power

    • heat of combustion: heat released when one mole of a substance in its standard state is oxidized to carbon dioxide and water

Sources of alkanes

Sources of Alkanes

  • Natural gas

    • 90-95% methane

  • Petroleum

    • gases (bp below 20°C)

    • naphthas, including gasoline (bp 20 - 200°C)

    • kerosene (bp 175 - 275°C)

    • fuel oil (bp 250 - 400°C)

    • lubricating oils (bp above 350°C)

    • asphalt (residue after distillation)

  • Coal



  • Octane rating:the percent 2,2,4-trimethylpentane (isooctane) in a mixture of isooctane and heptane that has equivalent antiknock properties

Synthesis gas

Synthesis Gas

  • A mixture of carbon monoxide and hydrogen in varying proportions which depend on the means by which it is produced

Synthesis gas1

Synthesis Gas

  • Synthesis gas is a feedstock for the industrial production of methanol and acetic acid

    • it is likely that industrial routes to other organic chemicals from coal via methanol will also be developed

Alkanes and cycloalkanes

Alkanes andCycloalkanes

End Chapter 2

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