2-2-11

ORGANIC ORGANIC CHEMISTRY 171CHEMISTRY 171

Section 201

2-2-22

AlkanesAlkanesandandCycloalkanesCycloalkanes

Chapter 2Chapter 2

2-2-33

StructureStructure Hydrocarbon:Hydrocarbon: a compound composed only of carbon and hydrogen

Saturated hydrocarbon:Saturated hydrocarbon: a hydrocarbon containing only single bonds

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

Aliphatic hydrocarbon:Aliphatic hydrocarbon: another name for an alkane

2-2-44

HydrocarbonsHydrocarbons

H-C C-HH-C-C-H

H

H

H

H HC C

H

H H

Hydrocarbons

Alkanes(Chapter 2)

Alkenes(Chapters 5-6)

Alkynes(Chapter 7)

Arenes(Chapter 21-22)

Only carbon-carbon single

bonds

One or more carbon-carbondouble bonds

One or morecarbon-carbontriple bonds

One or morebenzenelike

rings

Class

Example

Carbon-carbon

bonding

Name Ethane Ethene Acetylene Benzene

Saturated Unsaturated

2-2-55

Functional GroupsFunctional Groups

Functional group: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.

2-2-666

Functional Groups: HydrocarbonsHydrocarbons are compounds made up of only the elements carbon and hydrogen. They may be aliphatic or aromatic.

Organic Molecules and Functional Groups

2-2-777

Functional Groups: Heteroatoms

Organic Molecules and Functional Groups

2-2-888

Functional Groups: Carbonyl groups

2-2-99

AlcoholsAlcohols

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

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

H-C-C-O-H

H

H

H

H::-C-O-H

Ethanol(an alcohol)

Functionalgroup

CH3-CH2-OH CH3CH2OHor

2-2-1010

AlcoholsAlcohols

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.

CH3-C-OH CH3-C-OH

CH3

H

CH3-C-OH

CH3

CH3

A 1° alcohol A 2° alcohol A 3° alcoholH

H

2-2-1111

AlcoholsAlcohols

There are two alcohols with molecular formula C3H8O

CH3CH2CH2OH

CH3CHCH3

OH

H-C-C-C-O-H

H

H

H

H

H

H

C-C-C-H

H

H

OH

HHH

H

or

or

a 2° alcohol

a 1° alcohol

2-2-1212

AminesAmines

Contain an amino groupamino group; an sp3-hybridized nitrogen bonded to one, two, or three carbon atoms.• An amine may by 1°, 2°, or 3°.

CH3 N H

H

CH3 N H

CH3

CH3 N CH3

CH3

Methylamine(a 1° amine)

Dimethylamine(a 2° amine)

Trimethylamine(a 3° amine)

: : :

2-2-1313

Aldehydes and KetonesAldehydes and Ketones

Contain a carbonyl (C=O) carbonyl (C=O) group.group.

C H

O

Functionalgroup

Acetone(a ketone)

CH3-C-H CH3-C-CH3

O O

C

O

Functionalgroup

Acetaldehyde(an aldehyde)

::::

2-2-1414

Carboxylic AcidsCarboxylic Acids

Contain a carboxyl (-COOH) carboxyl (-COOH) group.group.

C O

O

H CH3-C-O-HO

CH3COOH CH3CO2H

: ::: or or

Acetic acid(a carboxylic acid)

Functionalgroup

2-2-1515

Carboxylic EstersCarboxylic Esters

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

C O

O

Functionalgroup

CH3-C-O-CH2-CH3

Ethyl acetate(an ester)

O

2-2-1616

Carboxylic AmideCarboxylic Amide

Carboxylic amideCarboxylic amide, commonly referred to as an amideamide: 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°.

CH3-C-N-H

HAcetamide(a 1° amide)

O

C N

O

Functionalgroup

2-2-1717

Alkanes

2-2-1818

StructureStructure

Shape• tetrahedral about carbon• all bond angles are approximately 109.5°

• sp3 hybridization

2-2-1919

Drawing AlkanesDrawing Alkanes Line-angle formulas

• an abbreviated way to draw structural formulas

• each vertex and line ending represents a carbon

CH3CH2CH2CH3CH3CH2CH3 CH3CH2CH2CH2CH3

PentaneButanePropane PentanePropane

Structuralformula

Line-angle formula

Ball-and-stickmodel

2-2-2020

Constitutional IsomerismConstitutional Isomerism Constitutional isomers:Constitutional isomers: compounds with the same molecular formula but a different connectivity of their atoms• example: C4H10

CH3CH2CH2CH3

Butane(bp -0.5°C)

CH3

CH3CHCH3

2-Methylpropane(bp -11.6°C)

2-2-2121

Constitutional IsomerismConstitutional 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

CH3CHCH2CHCH3 CH3

CH3

CH3CH2CHCHCH3

CH3

CH3

and (each is C7H16)

CH3CH2CHCHCH3

CH3

CH3

CH3CHCH2CHCH3

CH3

CH3

4

454

31and

5

1 12 23

3

52

5 4 3

2 1

2-2-2222

Constitutional IsomerismConstitutional Isomerism

World populationWorld populationis aboutis about6,000,000,0006,000,000,000

4,111,846,763

4,347

75

31

Constitutional Isomers

MolecularFormula

CH4

C5H12

C10H22

C15H32

C30H62

36,797,588C25H52

2-2-2323

Nomenclature - IUPACNomenclature - IUPAC Suffix -aneane specifies an alkaneane Prefix tells the number of carbon atoms

undec-dodec-

tetradec-pentadec-hexadec-heptadec-

nonadec-eicos-

tridec-

11121314151617

octadec- 181920

Prefixmeth-eth-prop-but-pent-hex-

oct-non-dec-

1234567hept-89

10

Carbons CarbonsPrefix

2-2-2424

Nomenclature - IUPACNomenclature - IUPAC Parent name:Parent name: the longest carbon chain Substituent:Substituent: a group bonded to the parent chain • alkyl groupalkyl group:: a substituent derived by removal of a hydrogen from an alkane; given the symbol R-Alkane Alkyl group

CH4

Name Name

Methane CH3- Methyl group

CH3CH3 Ethane CH3CH2- Ethyl group

2-2-2525

Nomenclature - IUPACNomenclature - IUPAC1.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

CH3CHCH3

CH3

2-Methylpropane

12 3

CH3CH2CH2CHCH3

CH3

2-Methylpentane

123

45 5

432

1

(not 4-methylpentane)

2-2-2626

Nomenclature - IUPACNomenclature - IUPAC5. 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

2,4-Dimethylhexane1

23

45

66

54

32

1

(not 3,5-dimethylhexane)

2-2-2727

Nomenclature - IUPACNomenclature - IUPAC6. 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

3-Ethyl-5-methylheptane

12

34

5 67 7

65

43 2

1

(not 3-methyl-5-ethylheptane)

2-2-2828

Nomenclature - IUPACNomenclature - IUPAC7. The prefixes di-, tri-, tetra-, etc. are not included in alphabetization

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

4-Ethyl-2,2-dimethylhexane(not 2,2-dimethyl-4-ethylhexane)

12 3

45

6

2-2-2929

Nomenclature - IUPACNomenclature - IUPAC Alkyl groups

1-methylethyl (isopropyl)

propyl

ethyl

methyl

CondensedStructural FormulaName

CH3

-CH2CH3

-CH3

-CH2CH2CH3

-CHCH3

1,1-dimethylethyl (tert-butyl)

1-methylpropyl (sec-butyl)

2-methylpropyl (isobutyl)

butyl

CH3

CH3

CH3

CH3

-CH2CH2CH2 CH3

-CH2CHCH3

-CHCH2CH3

-CCH3

CondensedStructural FormulaName

2-2-3030

Nomenclature - CommonNomenclature - 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

CH3CH2CH2CH2CH3

CH3CHCH3

CH3

CH3CH2CHCH3

CH3

CH3CH2CH2CH3

CH3CCH3

CH3

CH3

Pentane Isopentane

IsobutaneButane

Neopentane

2-2-3131

Classification of C & HClassification of C & H Primary (1°) C:Primary (1°) C: a carbon bonded to one other carbon• 1° H: a hydrogen bonded to a 1° carbon

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

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

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

2-2-3232

CycloalkanesCycloalkanes General formula CCnnHH2n2n

• 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-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

2-2-3333

CycloalkanesCycloalkanes Line-angle drawings

• each line represents a C-C bond• each vertex and line ending represents a C

C

C CC

CC

C

C H2C

H2CCH2

CH

CH2

CH

CH3

CH3

C8H16

2-2-3434

CycloalkanesCycloalkanes Example:Example: name these cycloalkanes

(a) (b)

(c) (d)

2-2-3535

IUPAC - GeneralIUPAC - General prefix-infix-suffix

• prefixprefix tells the number of carbon atoms in the parent

• infixinfix tells the nature of the carbon-carbon bonds

• suffixsuffix tells the class of compound

one or more triple bonds

one or more double bonds

all single bonds

-yn-

-en--an-

Nature of Carbon-Carbon Bonds in the Parent ChainInfix

Suffix Class

-e

-ol

-al

-one

-oic acid

hydrocarbon

alcohol

aldehyde

ketonecarboxylic acid

-amine amine

2-2-3636

IUPAC - GeneralIUPAC - Generalprop-enen-e = propeneeth-anan-ol = ethanolbut-anan-one = butanonebut-anan-al = butanalpent-anan-oic acid = pentanoic acidcyclohex-anan-ol = cyclohexanoleth-ynyn-e = ethyneeth-anan-amine = ethanamine

CH3CH=CH2

CH3CH2OH CH3CH2CH2CH2COH

O

HC CH

OH

CH3CH2CH2CH

O

CH3CCH2CH3

O

CH3CH2NH2

2-2-3737

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

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

2-2-3838

ConformationsConformations Staggered conformation: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

H

H H

H H

H

2-2-3939

ConformationsConformations Eclipsed conformation: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

H

H H

H

HH

2-2-4040

ConformationsConformations Torsional strain Torsional strain

• also called eclipsed interaction strain 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

+12.6 kJ/mol

2-2-4141

ConformationsConformations Dihedral angle Dihedral angle the angle created by two intersecting planes

2-2-4242

ConformationsConformations Steric strainSteric strain ((nonbonded interaction nonbonded interaction strain)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:Angle strain:• strain that arises when a bond angle is either compressed or expanded compared to its optimal value

2-2-4343

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

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

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

H

H

H

H

H

H

2-2-4444

CyclohexaneCyclohexane Chair conformation: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

2-2-4545

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

2-2-4646

CyclohexaneCyclohexane 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

2-2-4747

CyclohexaneCyclohexane Boat conformation: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

2-2-4848

CyclohexaneCyclohexane Twist-boat conformationTwist-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

2-2-4949

Equatorial and axial methyl conformations

MethylcyclohexaneMethylcyclohexane

CH3

CH3

+7.28 kJ/mol

2-2-5050

Cis,TransCis,Trans Isomerism Isomerism Stereoisomers: Stereoisomers: compounds that have

• the same molecular formula • the same connectivity • a different orientation of their atoms in space

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

2-2-5151

Cis,TransCis,Trans Isomers Isomers 1,2-Dimethylcyclopentane

CH3

trans-1,2-Dimethyl-cyclopentane

cis-1,2-Dimethyl-cyclopentane

H3CH3C CH3

CH3

H

CH3

H

H

HH

H

HH

H

H

CH3

H3C

H

HH

HH

H

2-2-5252

Cis,TransCis,Trans Isomerism Isomerism 1,4-Dimethylcyclohexane

trans-1,4-Dimethyl-cyclohexane

cis-1,4-Dimethyl-cyclohexane

H

H3C

CH3

H

H

H3C

H

CH3

CH3

H3C H3C

CH3

2-2-5353

Cis,TransCis,Trans Isomerism Isomerism trans-1,4-Dimethylcyclohexane

• the diequatorial-methyl chair conformation is more stable by approximately 2 x (7.28) = 14.56 kJ/molCH3

H

H

CH3

HH3C

CH3

H(more stable)(less stable)

2-2-5454

Cis,TransCis,Trans Isomerism Isomerism cis-1,4-Dimethylcyclohexane

conformations are of equal stability

H

CH3

H

CH3

H

H3C

H

CH3

2-2-5555

Physical PropertiesPhysical 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

2-2-5656

Physical PropertiesPhysical Properties Constitutional isomers have different physical properties

NameDensity (g/mL)

hexane2-methylpentane

3-methylpentane

2,3-dimethylbutane

2,2-dimethylbutane

68.7

60.3

63.358.0

49.7

-95

-154

-118-129

-98

0.659

0.653

0.6640.661

0.649

bp (°C)

mp (°C)

2-2-5757

Preparation and Preparation and Reactions of Reactions of

AlkanesAlkanes

2-2-5858

Preparation of Alkanes via Preparation of Alkanes via ReductionReduction

1) Hydrogenation of Alkenes

Hydrogenation of Alkynes

C C

R

R

R'

R'

H2/Pt CH CH

R

R

R'

R'

R C C R'H2/Pt R C

H

H

C

H

H

R'

2-2-5959

2.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

2-2-6060

CH3 CH3

CH3CH-Br + Mg CH3CH-MgBrisopropyl bromide isopropyl magnesium bromide

CH3

CH3CH-MgBr + H2O CH3CH2CH3

propane

CH3CH2CH2-MgBr + D2O CH3CH2CH2D heavy water

CH3 CH3

CH3CH-MgBr + D2O CH3CHD

2-2-6161

b) 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

Brtert-butyl bromide isobutane

SnCl2

2-2-6262

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.)

2-2-6363

Alkane, reactions:

1. Halogenation

2. Combustion (oxidation)

3. Pyrolysis (cracking)

2-2-6464

1.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

2-2-6565

CH3CH3 + Cl2, hv CH3CH2-Cl + HCl ethane ethyl chloride

CH3CH2CH3 + Cl2, hv CH3CH2CH2-Cl + CH3CHCH3

propane n-propyl chloride Cl

isopropyl chloride 45%

55%

gives a mixture of both the possible alkyl halides!

2-2-6666

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

2-2-6767

chlorination of propane, mechanism:

1) 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•

2-2-6868

2.Oxidation of Alkanes 2.Oxidation of Alkanes ( combustion )( combustion )

Oxidation is the basis for their use as energy sources for heat and power• heat of combustion:heat of combustion: heat released when one mole of a substance in its standard state is oxidized to carbon dioxide and water

CH4 2O2

CH3CH2CH3 5O2

CO2

3CO2 4H2O

2H2O+ + -890.4 (-212.8)

+ + -2220 (-530.6)

Methane

Propane

H0 kJ(kcal)/mol

2-2-6969

Sources of AlkanesSources 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

2-2-7070

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

Heptane(octane rating 0)

2,2,4-Trimethylpentane(octane rating 100)

2-2-7171

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

C + H2O heat CO + H2Coal

1+catalyst +

2CH4 2H2COO2

Methane

2-2-7272

Synthesis GasSynthesis 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

CH3OH + COcatalyst

CH3COHO

Methanol Acetic acid

+CO 2H2 CH3OHcatalyst

Methanol

2-2-7373

Alkanes andAlkanes andCycloalkanesCycloalkanes

End Chapter 2End Chapter 2