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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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Alkanes

and

Cycloalkanes

Chapter 2


Structure
Structure

  • 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 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



Alcohols
Alcohols

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

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


Alcohols1
Alcohols

  • 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.


Alcohols2
Alcohols

There are two alcohols with molecular formula C3H8O


Amines
Amines

  • 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°.



Structure1
Structure

  • 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

World population

is about

6,000,000,000


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


Cycloalkanes
Cycloalkanes

  • 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


Cycloalkanes1
Cycloalkanes

  • Line-angle drawings

    • each line represents a C-C bond

    • each vertex and line ending represents a C


Cycloalkanes2
Cycloalkanes

  • 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

Suffix

Class

Infix

-e

hydrocarbon

-an-

all single bonds

-ol

alcohol

-en-

one or more double bonds

-al

aldehyde

-yn-

one or more triple bonds

-amine

amine

-one

ketone

-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


Conformations
Conformations

  • 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


Conformations1
Conformations

  • 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


Conformations2
Conformations

  • 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


Conformations3
Conformations

  • 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


Conformations4
Conformations

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


Conformations5
Conformations

  • 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


Cyclopropane
Cyclopropane

  • 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


Cyclohexane
Cyclohexane

  • 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


Cyclohexane1
Cyclohexane

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


Cyclohexane2
Cyclohexane

  • 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


Cyclohexane3
Cyclohexane

  • 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


Cyclohexane4
Cyclohexane

  • 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


Methylcyclohexane
Methylcyclohexane

  • 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 of alkanes via reduction

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

  • SB SA WA WB

  • CH3CH2CH2-Br + Mg  CH3CH2CH2-MgBr

  • n-propyl bromide n-propyl magnesium bromide

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

  • propane


CH3 CH3

CH3CH-Br + Mg  CH3CH-MgBr

isopropyl bromide isopropyl magnesium bromide

CH3

CH3CH-MgBr + H2O  CH3CH2CH3

propane

CH3CH2CH2-MgBr + D2O  CH3CH2CH2D

heavy water

CH3 CH3

CH3CH-MgBr + D2O  CH3CHD


  • 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 chloride n-butane

  • CH3 CH3

  • CH3CCH3 + Zn/H+  CH3CHCH3 + ZnBr2

  • Br

  • tert-butyl bromide isobutane

SnCl2


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.)



  • 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%

55%

gives a mixture of both the possible

alkyl halides! 


CH3CH2CH2CH3 + Cl2, hv  CH3CH2CH2CH2-Cl

n-butane n-butyl chloride 28%

+

CH3CH2CHCH3 72%

Cl

sec-butyl chloride

CH3 CH3

CH3CHCH3 + Cl2, hv  CH3CHCH2-Cl 64%

isobutane isobutyl chloride

+

CH3

CH3CCH3 36%

Cl

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
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


Gasoline
Gasoline

  • 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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