Chapter 2 Introduction to Hydrocabons Carbon Backbone, Nomenclature, Physical & Chemical Properties.

Chapter 2

Introduction toHydrocabons

Carbon Backbone, Nomenclature, Physical &

Chemical Properties

HYDROCARBONS• Compounds composed of only carbon and hydrogen atoms

(C, H). Each carbon has 4 bonds.

• They represent a “backbone” when other “heteroatoms” (O, N, S, .....) are substituted for H. (The heteroatoms give function to the molecule.)

• Acyclic (without rings); Cyclic (with rings); Saturated: only carbon-carbon single bonds; Unsaturated: contains one or more carbon-carbon double and/or triple bond

HYDROCARBONS• Alkanes contain only single ( ) bonds and have the

generic molecular formula: [CnH2n+2]

• Alkenes also contain double ( + ) bonds and have the generic molecular formula: [CnH2n]

• Alkynes contain triple ( + 2) bonds and have the generic molecular formula: [CnH2n-2]

• Aromatics are planar, ring structures with alternating single and double bonds: eg. C6H6

Types of Hydrocarbons

Each C atom is trigonal planar with sp2 hybridized orbitals.There is no rotation about the C=C bond in alkenes.

Each C atom is tetrahedral with sp3 hybridized orbitals. They only have single bonds.

Types of Hydrocarbons

Each C atom is linear with sp hybridized orbitals.

Each C--C bond is the same length; shorter than a C-C bond: longer than a C=C bond.The concept of resonance is used to explain this phenomena.

Propane

It is easy to rotate about the C-C bond in alkanes.

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Naming AlkanesNaming AlkanesCC11 - C - C10 10 : the number of C atoms present in the chain.

Each member CC33 - C - C1010 differs by one CH2 unit. This is called a homologous series.

Methane to butane are gases at normal pressures.Pentane to decane are liquids at normal pressures.

Nomenclature of Alkyl Substituents

Examples of Alkyl Substituents

Constitutional or structural isomers have the same molecular formula, but their atoms are linked differently. Naming has to account for them.

A compound can have more than one name, but a name must unambiguously specify only one compound

C7H16 can be any one of the following:

Alkanes (Different types of sp3 carbon atoms)

• Primary, 1o, a carbon atom with 3 hydrogen atoms: [R-CH3]

• Secondary, 2o, a carbon atom with 2 hydrogen atoms: [R-CH2-R]

• Tertiary, 3o, a carbon atom with 1 hydrogen atom: [R-CH-R] R

• Quaternary, 4o, a carbon atom with 0 hydrogen atoms: CR4

Different Kinds of sp3 Carbons and Hydrogens

Nomenclature of Alkanes

1. Determine the number of carbons in the parent hydrocarbon

CH3CH2CH2CH2CHCH2CH2CH3

CH3

12345678

CH3CH2CH2CH2CHCH2CH3

CH2CH2CH3

45678

123

CH3CH2CH2CHCH2CH2CH3

CH2CH2CH2CH3

1234

5 6 7 8

2. Number the chain so that the substituent gets the lowest possible number

CH3CHCH2CH2CH3

CH3

1 2 3 4 5

2-methylpentane

CH3CH2CH2CHCH2CH2CH2CH3

CHCH3

CH3

1 2 3 4 5 6 7 8

4-isopropyloctane

CH3CHCH2CH2CH3

CH3

common name: isohexanesystematic name: 2-methylpentane

3. Number the substituents to yield the lowest possible number in the number of the compound

CH3CH2CHCH2CHCH2CH2CH3

CH3 CH2CH3 5-ehtyl-3-methyloctanenot

4-ethyl-6-methyloctanebecause 3<4

(substituents are listed in alphabetical order)

4. Assign the lowest possible numbers to all of the substituents

CH3CH2CHCH2CHCH3

CH3CH3

2,4-dimethylhexane

CH3CH2CH2C

CH3

CH3

CCH 2CH 3

CH3

CH3

3,3,4,4-tetramethylheptane

CH3CH2CHCH2CH2CHCHCH2CH2CH3

CH2CH3

CH2CH3 CH2CH3

CH3

3,3,6-triethyl-7-methyldecane

5. When both directions lead to the same lowest number for oneof the substituents, the direction is chosen that gives the lowest possible number to one of the remaining substituents

CH3CHCH2CHCH3

CH3

CH3 CH3

2,2,4-trimethylpentanenot

2,4,4-trimethylpentanebecause 2<4

CH3CH2CHCHCH2CHCH2CH3

CH3

CH3 CH2CH3

6-ethyl-3,4-dimethyloctanenot

3-ethyl-5,6-dimethyloctanebecause 4<5

6. If the same number is obtained in both directions, the firstgroup receives the lowest number

CH3CH2CHCH2CHCH2CH3

CH3

CH2CH3

3-ethyl-5-methylheptanenot

5-ethyl-3-methylheptane

CH3CH2CHCH3

Cl

Br

2-bromo-3-chlorobutanenot

3-bromo-2-chlorobutane

7. In the case of two hydrocarbon chains with the same number ofcarbons, choose the one with the most substituents

CH3CH2CHCH2CH2CH3

CHCH3

CH31

2

3 4 5 6

3-ethyl-2-methylhexane (two substituents)

CH3CH2CHCH2CH2CH3

CHCH3

CH3

1 2 3 4 5 6

3-isopropylhexane (one substituent)

8. Certain common nomenclatures are used in the IUPAC system

CH3CH2CH2CH2CHCH2CH2CH3

CHCH3

CH3

4-isopropyloctaneor

4-(1-methylethyl)octane

CH3CH2CH2CH2CHCH2CH2CH2CH2CH3

CH2CHCH3

CH3

5-isobutyldecaneor

5-(2-methylpropyl)decane

CCnnHH22nn

Cycloalkane Nomenclature

Cycloalkanes• Cycloalkanes are alkanes that contain a

ring of three or more carbons.• Count the number of carbons in the ring,

and add the prefix cyclo to the IUPAC name of the unbranched alkane that has that number of carbons.

CyclopentaneCyclopentane CyclohexaneCyclohexane

EthylcyclopentaneEthylcyclopentane

CHCH22CHCH33

• Name any alkyl groups on the ring in the usual way. A number is not needed for a single substituent.

Cycloalkanes

• Name any alkyl groups on the ring in the usual way. A number is not needed for a single substituent.

• List substituents in alphabetical order and count in the direction that gives the lowest numerical locant at the first point of difference.

3-Ethyl-1,1-dimethylcyclohexane3-Ethyl-1,1-dimethylcyclohexane

CHCH22CHCH33

HH33CC CHCH33

Cycloalkanes

For more than two substituents,

CH3CH2CH2

H3C CH2CH3

4-ethyl-2-methyl-1-propylcyclohexanenot

1-ethyl-3-methyl-4-propylcyclohexanebecause2<3

not 5-ethyl-1-methyl-2-propylcyclohexane

because 4<5

CH3

CH3

CH3

1,1,2-trimethylcyclopentanenot

1,2,2-trimethylcyclopentanebecause1<2

not1,1,5-trimethylcyclopentane

because 2<5

2.17Physical Properties of

Alkanesand Cycloalkanes

Crude oilCrude oil

Refinery gasRefinery gasRefinery gasRefinery gas

CC11-C-C44

Light gasolineLight gasoline(bp: 25-95 °C)(bp: 25-95 °C)

Light gasolineLight gasoline(bp: 25-95 °C)(bp: 25-95 °C)

CC55-C-C1212

NaphthaNaphtha(bp 95-150 °C)(bp 95-150 °C)

NaphthaNaphtha(bp 95-150 °C)(bp 95-150 °C)

KeroseneKerosene(bp: 150-230 °C)(bp: 150-230 °C)

KeroseneKerosene(bp: 150-230 °C)(bp: 150-230 °C)

CC1212-C-C1515

Gas oilGas oil(bp: 230-340 °C)(bp: 230-340 °C)

Gas oilGas oil(bp: 230-340 °C)(bp: 230-340 °C)

CC1515-C-C2525

ResidueResidueResidueResidue

Fig. 2.15

Example of Intramolecular Forces: Protein Folding

10-40kJ/mol

700-4,000kJ/mol

150-1000kJ/mol

0.05-40kJ/mol

Ion-dipole(Dissolving)40-600kJ/mol

Ion-Dipole Forces (40-600 kJ/mol)• Interaction between an ion and a dipole (e.g. NaOH and

water = 44 kJ/mol)• Strongest of all intermolecular forces.

Intermolecular ForcesIntermolecular Forces

Ion-Dipole & Dipole-Dipole Interactions: like dissolves like

• Polar compounds dissolve in polar solvents & non-polar in non-polar

Dipole-Dipole Forces

(permanent dipoles)


5-25 kJ/mol

Dipole-Dipole Forces


Boiling Points &

Hydrogen Bonding

Hydrogen Bonding

• Hydrogen bonds, a unique dipole-dipole (10-40 kJ/mol).

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Hydrogen Bonding



DNA: Size, Shape & Self Assemblyhttp://www.umass.edu/microbio/chime/beta/pe_alpha/atlas/atlas.htm

Views & Algorithms

10.85 Å10.85 Å

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London or Dispersion Forces• An instantaneous dipole can induce another dipole in an

adjacent molecule (or atom).• The forces between instantaneous dipoles are called

London or Dispersion forces ( 0.05-40 kJ/mol).


van der Waals Forces

The boiling point of a compound increases with the increase in van der Waals force..and the Gecko!

Gecko: toe, setae, spatulae6000x Magnification

http://micro.magnet.fsu.edu/primer/java/electronmicroscopy/magnify1/index.html

Geim, Nature Materials (2003) Glue-free Adhesive100 x 10 6 hairs/cm2

Full et. al., Nature (2000)5,000 setae / mm2

600x frictional force; 10-7 Newtons per seta






Boiling Points of Alkanes

• governed by strength of intermolecular attractive forces

• alkanes are nonpolar, so dipole-dipole and dipole-induced dipole forces are absent

• only forces of intermolecular attraction are induced dipole-induced dipole forces

Induced dipole-Induced dipole Attractive Forces

++––++

––

• two nonpolar molecules• center of positive charge and center

of negative charge coincide in each

++––++

––

• movement of electrons creates an instantaneous dipole in one molecule (left)


++––++––

• temporary dipole in one molecule (left) induces a complementary dipole in other molecule (right)


++––++ ––

• temporary dipole in one molecule (left) induces a complementary dipole in other molecule (right)


++––++ ––

• the result is a small attractive force between the two molecules


++–– ++ ––

• the result is a small attractive force between the two molecules


Boiling Points

•Increase with increasing number of carbons

• more atoms, more electrons, more opportunities for induced dipole-induceddipole forces

•Decrease with chain branching

• branched molecules are more compact withsmaller surface area—fewer points of contactwith other molecules

London Dispersion Forces


Which has the higherattractive force?


• more atoms, more electrons, more opportunities for induced dipole-induceddipole forces

HeptaneHeptanebp 98°Cbp 98°C

OctaneOctanebp 125°Cbp 125°C

NonaneNonanebp 150°Cbp 150°C

Boiling Points


• branched molecules are more compact withsmaller surface area—fewer points of contactwith other molecules

Octane: bp 125°COctane: bp 125°C

2-Methylheptane: bp 118°C2-Methylheptane: bp 118°C

2,2,3,3-Tetramethylbutane: bp 107°C2,2,3,3-Tetramethylbutane: bp 107°C

Boiling Points

•All alkanes burn in air to givecarbon dioxide and water.

2.18Chemical Properties:

Combustion of Alkanes

4817 kJ/mol4817 kJ/mol





HeptaneHeptane

OctaneOctane

NonaneNonane

Heats of Combustion

What pattern is noticed in this case?


• more moles of O2 consumed, more moles

of CO2 and H2O formed

Heats of Combustion





5 kJ/mol5 kJ/mol

8 kJ/mol8 kJ/mol

6 kJ/mol6 kJ/mol

Heats of Combustion

What pattern is noticed in this case?

8CO8CO22 + 9H + 9H22OO

5452 kJ/mol5452 kJ/mol5458 kJ/mol5458 kJ/mol


5466 kJ/mol5466 kJ/molOO22++ 2525

22

OO22++ 2525

22 OO22++ 2525

22 OO22++ 2525

22

Figure 2.17Figure 2.17


• more moles of O2 consumed, more moles

of CO2 and H2O formed


• branched molecules are more stable(have less potential energy) than theirunbranched isomers

Heat of CombustionPatterns

•Isomers can differ in respect to their stability.

•Equivalent statement:

–Isomers differ in respect to their potential energy.

Important Point

Differences in potential energy can be measured by comparing heats of combustion. (Worksheet problems)

2.19Oxidation-Reduction in Organic

ChemistryOxidation of a carbon atom corresponds to an increase in the number of bonds to the carbon atom and/or a decrease in the number of hydrogens bonded to the carbon atom. See examples on the board.

increasing oxidation increasing oxidation state of carbonstate of carbon

-4-4 -2-2 00 +2+2 +4+4

HH

HH

HH

CC HH

HH

HH

HH

CC OOHH

OO

CCHHHH

OO

CCOOHHHH

OO

CCOOHHHHOO

increasing oxidation increasing oxidation state of carbonstate of carbon

-3-3 -2-2 -1-1

HCHC CHCH

CC CC

HH

HH HH

HH

CC CC HH

HHHH

HH HH

HH

• But most compounds contain several (or many)carbons, and these can be in different oxidationstates.

CHCH33CHCH22OHOH CC22HH66OO

• But most compounds contain several (or many)carbons, and these can be in different oxidationstates.

• Working from the molecular formula gives the average oxidation state.


Average oxidationAverage oxidationstate of C = -2state of C = -2

• How can we calculate the oxidation stateof each carbon in a molecule that containscarbons in different oxidation states?


Average oxidationAverage oxidationstate of C = -2state of C = -2

Table 2.5 How to Calculate Oxidation Numbers

• 1. Write the Lewis structure and include unshared electron pairs.

HH

CC

HH

HH

HH

OO

HH

CC HH••••

••••


• 2. Assign the electrons in a covalent bond between two atoms to the more electronegative partner.

HH

OO

HH

CC

HH

HH

HH

CC HH••••••••

••••••••

••••

••••

••••

••••••••

• 3. For a bond between two atoms of the same element, assign the electrons in the bond equally.

HH

OO

HH

CC

HH

HH

HH

CC HH••••••••

••••••••

••••

••••

••••

••••••••


• 3. For a bond between two atoms of the same element, assign the electrons in the bond equally.

HH

OO

HH

CC

HH

HH

HH

CC HH••••••••

••••••••

••••

••••

••••

•••••••• ••••


• 4. Count the number of electrons assigned to each atom and subtract that number from the number of valence electrons in the neutral atom; the result is the oxidation number.

HH

OO

HH

CC

HH

HH

HH

CC HH••••••••

••••••••

••••

••••

••••

•••••••• ••••

Each H Each H == +1+1C of CHC of CH33 == -3-3

C of CHC of CH22O O == -1-1

O O == -2-2


XX YY

XX less electronegative than carbon less electronegative than carbon

YY more electronegative than carbon more electronegative than carbon

oxidationoxidation

reductionreductionCC CC

Generalization

Oxidation of carbon occurs when a bond between carbon and an atom which is less electronegative than carbon is replaced by a bond to an atom that is more electronegative than carbon. The reverse process is reduction.

CHCH33ClCl HHClClCHCH44 ClCl22++ ++

OxidationOxidation

++ 2Li2Li LiLiClClCHCH33ClCl CHCH33LiLi ++

ReductionReduction

Examples

Chapter 4

Alcohols & Halides

Functions, Nomenclature,

Common Functional GroupsCommon Functional Groups

Class General Formula

Halohydrocarbons RX

Alcohols R

Ethers RR

Amines

R-NH2

Nomenclature of Alkyl Halides

CH3Cl CH3CH2FCH3CHI

CH3

CH3CH2CHBr

CH3chloromethane fluoroethane2-iodopropane 2-bromobutane

In the IUPAC system, alkyl halides are named as substituted alkanes

CH3CH2CHCH2CH2CH2CH3

CH3

Br

2-bromo-5-methylheptane

CH3CH2CHCH2CH2CH2Cl

CH3

CH3

1-chloro-5,5-dimethylhexane

CH2CH3

I

1-ethyl-2-iodocyclopentane

Br

Cl

CH3

4-bromo-2-chloro-1-methylcyclohexane

Structures of Alkyl Halides

Different Kinds of Alkyl Halides

Nomenclature of Ethers

CH3CHOCHCH2CH3

CH3

CH3

sec-butyl isopropyl ether

CH3CH2CH2CH2O

cyclohexyl isopentyl ether

CH3O

methoxy

CH3CH2O

ethoxy

CH3CH2O

CH3

isopropoxy

As substituents:

CH3CH2CHO

CH3

sec-butoxy

CH3CH2O

CH3

CH3

tert-butoxy

???

Structures of Alcohol and Ether

Nomenclature of Alcohols• In an alcohol, the OH is a functional group

• A functional group is the center of reactivity in a molecule

1. Determine the parent hydrocarbon containing the functional group

CH3CHCH2CH3

OH

1 2 3 4

2-butanolor

butan-2-ol

CH3CH2CH2CHCH2OH

CH2CH3

12345

2-ethyl-1-pentanolor

2-ethylpentan-1-ol

CH3CH2CH2CH2OCH2CH2CH2OH

123

3-butoxy-1-propanolor

3-butoxypropan-1-ol

2. The functional group suffix should get the lowest number

HOCH 2CH 2CH 2Br

1 2 3

3-bromo-1-propanol

ClCH2CH2CHCH3

OH

1234

4-chloro-2-butanol

CH3CCH 2CHCH 3

CH3

CH3 OH

12345

4,4-dimethyl-2-pentanol

3. When there is both a functional group suffix and a substituent,the functional group suffix gets the lowest number

CH3CHCHCH2CH3

Cl OH

2-chloro-3-pentanolnot

4-chloro-3-pentanol

CH3CH2CH2CHCH2CHCH3

CH3OH

2-methyl-4-heptanolnot

6-methyl-4-heptanol

CH3

OH

3-methylcyclohexanolnot

5-methylcyclohexanol

4. If there is more than one substituent, the substituents are citedin alphabetical order

CH3CHCH2CHCH2CHCH3

Br

CH2CH3

OH

6-bromo-4-ethyl-2-heptanol

CH2CH3

OHH3C

2-ethyl-5-methylcyclohexanol

CH3

HO

CH3

3,4-dimethylcyclopentanol

Nomenclature of Amines

CH3CH2CH2CH2NH2

1234

1-butanamineor

butan-1-amine

CH3CH2CHCHCH2CH3

NHCH2CH3

1 2 3 4 5 6

N-ethyl-3-hexamineor

N-ethylhexan-3-amine

CH3CH2CH2NCH 2CH 3

CH3

123

N-ethyl-N-methyl-1-propanamineor

N-ethyl-N-methylpropan-1-amine

• The substituents are listed in alphabetical order and a number or an “N” is assigned to each one

CH3CHCH2CH2NCH 3

Cl

1234

3-chloro-N-methyl-1-butanamine

CH3CH2CHCHCHCH3

NHCH2CH3

CH31 2 3 4 5 6

N-ethyl-5-methyl-3-hexanamine

CH3CHCHCHCH3

NH3C CH3

Br12345

4-bromo-N,N-dimethyl-2-pentanamine

CH2CH3

NHCH2CH2CH3

2-ethyl-N-propylcyclohexanamine

Structures of Amines

Naming Quaternary Ammonium Salts

N+

CH3

H3C CH3

CH3

HO-

tetramethylammonium hydroxide

N+

CH3

CH3CH2CH2 CH3

CH3

Cl-

ethyldimethylpropylammonium chloride

OtherOther Common Functional GroupsCommon Functional Groups

Class General Formula

Aldehydes

Ketones

Carboxylic Acids

Esters

Amides

R-C-HO

R-C-R'O

R-C-OHO

R-C-OR'O

R-C-NO

R

"

R

'

• The greater the attractive intermoleclar forces between molecules, the higher is the boiling point of the compound, eg. water.

Attractive Forces

van der Waals force

Dipole–dipole interaction

Hydrogen bonds

Covalent bonds

Ionic bonds

Ion-dipoleDispersionForces

Protein Shape: Forces, Bonds, Self Assembly,Folding (Intramolecular forces)

10-40kJ/mol

700-4,000kJ/mol

150-1000kJ/mol

0.05-40kJ/mol

Ion-dipole(Dissolving)40-600kJ/mol

• A hydrogen bond is a special kind of dipole–dipole interaction

Dipole–Dipole Interaction

Dipole–dipole interactions are stronger than van der Waals force but weaker than ionic or covalent bonds

van der Waals Forces

The boiling point of a compound increases with the increase in van der Waals force

Ion-Dipole & Dipole-Dipole Interactions: like dissolves like

• Polar compounds dissolve in polar solvents & non-polar in non-polar

Chapter 3

Alkanes & Cycloalkane

Conformations

Conformations of Alkanes: Rotation about Carbon–Carbon Bonds

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Conformational AnalysisDrawing Acyclic Molecules

• Newman Projections

Conformational Analysis Drawing Acyclic Molecules

• Sawhorse Drawings

• Torsional strain: repulsion between pairs of bonding electrons

• A staggered conformer is more stable than an eclipsed conformer

Different Conformations of Ethane

Conformations of n-Butane• Steric strain: repulsion between the electron clouds of atoms or groups

Cycloalkanes: Ring Strain

• Angle strain results when bond angles deviate from the ideal 109.5° bond angle

The Shapes of Cycloalkanes:Planar or Nonplanar?

•Assumed cycloalkanes were planar Assumed cycloalkanes were planar polygons.polygons.

•Believed distortion of bond angles from Believed distortion of bond angles from 109.5° 109.5° gives angle strain to some cycloalkanes.gives angle strain to some cycloalkanes.

• One for two is great in baseball.One for two is great in baseball.

Adolf von Baeyer (19th century)

Types of Strain

• • Torsional strain strain that results from eclipsed bonds (measure of the dihedral angle)

• • Van der Waals strain or (Steric strain)strain that results from atoms being too close together.

• • Angle strain results from distortion of bond angles from normal values, for a

tetrahedron 109.5o

Measuring Strain in Cycloalkanes

•Heats of combustion can be used to compareHeats of combustion can be used to comparestabilities of isomers.stabilities of isomers.

•But cyclopropane, cyclobutane, etc. are not isomers.But cyclopropane, cyclobutane, etc. are not isomers.

•All heats of combustion increase as the numberAll heats of combustion increase as the numberof carbon atoms increase.of carbon atoms increase.

Measuring Strain in Cycloalkanes

•Therefore, divide heats of combustion by numberTherefore, divide heats of combustion by number of carbons and compare heats of combustion of carbons and compare heats of combustion on a "per CHon a "per CH22 group" basis. group" basis.

•CycloalkaneCycloalkane kJ/molkJ/mol Per CHPer CH22

•CyclopropaneCyclopropane 2,0912,091 697697•CyclobutaneCyclobutane 2,7212,721 681681•CyclopentaneCyclopentane 3,2913,291 658658•CyclohexaneCyclohexane 3,9203,920 653653•CycloheptaneCycloheptane 4,5994,599 657657•CyclooctaneCyclooctane 5,2675,267 658658•CyclononaneCyclononane 5,9335,933 659659•CyclodecaneCyclodecane 6,5876,587 659659

Heats of Combustion in Cycloalkanes

•CycloalkaneCycloalkane kJ/molkJ/mol Per CHPer CH22

•According to Baeyer, cyclopentane shouldAccording to Baeyer, cyclopentane should•have less angle strain than cyclohexane.have less angle strain than cyclohexane.•CyclopentaneCyclopentane 3,2913,291 658658•CyclohexaneCyclohexane 3,9203,920 653653

•The heat of combustion per CHThe heat of combustion per CH22 group is group is

•less for cyclohexane than for cyclopentane.less for cyclohexane than for cyclopentane.•Therefore, cyclohexane has less strain Therefore, cyclohexane has less strain thanthan•cyclopentane.cyclopentane.

Heats of Combustion in Cycloalkanes

•Heat of combustion suggests that angle strain is unimportant in cyclohexane.

•Tetrahedral bond angles require nonplanar geometries.

• The chair and boat conformations.

Conformations of Cyclohexane

• The chair conformation of cyclohexane is free of strain

• All of the bonds are staggered and the bond angles at carbon are close to tetrahedral.

Chair is the most stable conformation of

cyclohexane

• All of the bond angles are close to tetrahedral but close contact between flagpole hydrogens causes strain in boat.

180 pm180 pm

Boat conformation is less stable than the chair

• Eclipsed bonds bonds gives torsional strain to boat.

Boat conformation is less stable than the chair

• Less van der Waals strain and less torsional strain in skew boat.

BoatBoat Skew or Twist BoatSkew or Twist Boat

Skew boat is slightly more stable than boat

•The chair conformation of cyclohexane is themost stable conformation and derivativesof cyclohexane almost always exist in the chair conformation

Generalization

Axial and Equatorial Bonds in Cyclohexane

Drawing Cyclohexane

The 12 bonds to the ring can be divided into two sets of 6.

Axial bonds point "north and south"Axial bonds point "north and south"

6 Bonds are axial

The 12 bonds to the ring can be divided into two sets of 6.

Equatorial bonds lie along the equatorEquatorial bonds lie along the equator

6 Bonds are equatorial

Conformational Inversion

(Ring-Flipping) in Cyclohexane

• chair-chair interconversion (ring-flipping)

•rapid process (activation energy = 45 kJ/mol)

•all axial bonds become equatorial and vice versa

Conformational Inversion

Half-Half-chairchair

Half-Half-chairchair

SkewSkewboatboat

45 45 kJ/molkJ/mol

45 45 kJ/molkJ/mol

23 23 kJ/molkJ/mol

The Conformations of Cyclohexane and Their Energies

•most stable conformation is chair

•substituent is more stable when equatorial

Conformational Analysis of

Monosubstituted Cyclohexanes

Steric Strain of 1,3-Diaxial Interaction in Methylcyclohexane

5%5% 95%95%

• Chair chair interconversion occurs, but at any instant 95% of the molecules have their methyl group equatorial.

• An axial methyl group is more crowded than an equatorial one.

Methylcyclohexane

CHCH33

CHCH33

axialaxial

equatorialequatorial

5%5% 95%95%• Hydrogen atoms closer than 2.4 Angstroms will cause

steric strain.• This is called a "1,3-diaxial repulsion" a type of van

der Waals strain or Steric strain.

Methylcyclohexane

40%40% 60%60%

• Crowding is less pronounced with a "small" substituent such as fluorine.

• Size of substituent is related to its branching.

Fluorocyclohexane

FF

FF

Less than 0.01%Less than 0.01% Greater than 99.99%Greater than 99.99%

• Crowding is more pronounced with a "bulky" substituent such as tert-butyl.

• tert-Butyl is highly branched.

tert-Butylcyclohexane

C(CHC(CH33))33

C(CHC(CH33))33

van der Waalsvan der Waalsstrain due tostrain due to1,3-diaxial1,3-diaxialrepulsionsrepulsions

tert-Butylcyclohexane

Keq = [equatorial conformer]/[axial conformer]

• The larger the substituent on a cyclohexane ring, the more the equatorial substituted conformer will be favored

Disubstituted Cyclohexanes

Cis-trans Isomerism

Cyclic Alkanes StereochemistryCis -Trans Isomers

H

CH3H

CH3 cis-1,4-dimethylcyclohexane

H

H3C

CH3

Hring-flip

The Chair Conformers of cis-1,4-Dimethylcyclohexane

1,2-disubstituted-cis-cyclohexaneStereochemistry

CH3

CH3

CH3

CH3

CH3

CH3

Mirror

Same

(Rotate to See)

axial axial

equatorialequatorial

Cyclohexane StereochemistryDrawings: Cis isomers & the need for perspective

CH3CH3

CH3

CH3CH3

CH3

Are the methyl groups axial or equatorial?Are the methyl groups axial or equatorial?What is the actual conformational shape of the cyclohexane ring?What is the actual conformational shape of the cyclohexane ring?

The Chair Conformers of trans-1,4-Dimethylcyclohexane

H

CH3H3C

Htrans-1,4-dimethylcyclohexane

CH3

H

CH3

Hring-flip

Cyclohexane StereochemistryTrans isomers

CH3

CH3CH3

CH3 CH3

CH3

No Plane of Symmetry No plane of symmetry

Plane of Symmetry

CH3CH3

1-tert-Butyl-3-Methylcyclohexane

Cyclohexane StereochemistryCis -Trans Isomers

Position cis trans

1,2 e,a or a,e e,e or a,a

1,3

1,4

Complete the Table: a = axial; e = equatorial Complete the Table: a = axial; e = equatorial

e,a or a,e e,e or a,a

e,e or a,a a,e or e,a

Conformations of Fused Rings

• Trans-fused cyclohexane ring is more stable than cis-fused cyclohexane ring

Chapter 2 Introduction to Hydrocabons Carbon Backbone, Nomenclature, Physical & Chemical Properties.

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Transcript of Chapter 2 Introduction to Hydrocabons Carbon Backbone, Nomenclature, Physical & Chemical Properties.