Organic chemistry 171
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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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Section 201




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 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 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 Molecules and Functional Groups

Functional Groups: Heteroatoms

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

  • Contain a carbonyl (C=O) group.

Carboxylic Acids

  • Contain a carboxyl (-COOH) group.

Carboxylic Esters

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

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°.



  • Shape

    • tetrahedral about carbon

    • all bond angles are approximately 109.5°

    • sp3 hybridization

Drawing Alkanes

  • Line-angle formulas

    • an abbreviated way to draw structural formulas

    • each vertex and line ending represents a carbon

Constitutional Isomerism

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

    • example: C4H10

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 Isomerism

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Nomenclature - IUPAC

  • Suffix -ane specifies an alkane

  • Prefix tells the number of carbon atoms

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 - 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 - 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 - 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 - 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 - IUPAC

  • Alkyl groups

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

  • 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

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 - 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

  • 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

  • 1,2-Dimethylcyclopentane

Cis,Trans Isomerism

  • 1,4-Dimethylcyclohexane

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 Isomerism

  • cis-1,4-Dimethylcyclohexane

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 Properties

  • Constitutional isomers have different physical properties

Preparation and Reactions of Alkanes

Preparation of Alkanes via Reduction

1) Hydrogenation of Alkenes

Hydrogenation of Alkynes

  • 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


CH3CH-Br + Mg  CH3CH-MgBr

isopropyl bromide isopropyl magnesium bromide


CH3CH-MgBr + H2O  CH3CH2CH3



heavy water



  • 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


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.)

  • Alkane, reactions:

  • Halogenation

  • 2. Combustion (oxidation)

  • 3. Pyrolysis (cracking)

  • 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

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! 

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

  • 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 )

  • 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

  • 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

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

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 andCycloalkanes

End Chapter 2

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