chapter 03 alkanes and cycloalkanes -...

©2016 Gregory R Cook

cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24

0

Chapter 03 Alkanes and Cycloalkanes

CHEM 240: Fall 2016

Prof. Greg Cook

cook.chem.ndsu.nodak.edu/chem240

http://cook.chem.ndsu.nodak.edu/chem341



cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24

0Hydrocarbons

• Aliphatic hydrocarbons

• Alkanes: C-C single bondsAlkenes: C-C double bondsAlkynes: C-C triple bonds

• Aromatic hydrocarbons

• Arenes

2



cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24

0Source of Alkanes

• Most hydrocarbons come from crude oil

• Cracking breaks larger hydrocarbons into smaller, lighter hydrocarbons

• Reforming converts alkanes into highly branched and aromatic hydrocarbons

3



cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24

0Linear Alkanes

4

Alkanes CnH2n+2

MF CondensedName

Methane CH4CH4

Ethane C2H6

Propane

Butane

Pentane

Hexane

Heptane

Octane

Nonane

Decane

CH3CH3

C3H8 CH3CH2CH3

C4H10 CH3CH2CH2CH3

C10H22 CH3CH2CH2CH2CH2CH2CH2CH2CH2CH3

C9H20 CH3CH2CH2CH2CH2CH2CH2CH2CH3

C8H18 CH3CH2CH2CH2CH2CH2CH2CH3

C7H16 CH3CH2CH2CH2CH2CH2CH3

C6H14 CH3CH2CH2CH2CH2CH3

C5H12 CH3CH2CH2CH2CH3



cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24

0Branched and Cyclic Alkanes

5



cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24

0

Alkanes and Alkyl Groups: Isomers



cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24

0Linear Alkanes

7

Alkanes CnH2n+2

MF CondensedName

Methane CH4CH4

Ethane C2H6

Propane

Butane

Pentane

Hexane

Heptane

Octane

Nonane

Decane

CH3CH3

C3H8 CH3CH2CH3

C4H10 CH3CH2CH2CH3

C10H22 CH3CH2CH2CH2CH2CH2CH2CH2CH2CH3

C9H20 CH3CH2CH2CH2CH2CH2CH2CH2CH3

C8H18 CH3CH2CH2CH2CH2CH2CH2CH3

C7H16 CH3CH2CH2CH2CH2CH2CH3

C6H14 CH3CH2CH2CH2CH2CH3

C5H12 CH3CH2CH2CH2CH3



cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24

0Alkanes

8

• Linear Alkanes - carbons linked in a straight chain - also called normal alkanes.

• Branched Alkanes - some carbons are attached as a branch off the main chain

Constitutional Isomers of Butane C4H10

CH3 CH CH3

CH3

CH3 CH2 CH2 CH3

normal butanebp -0.4°C

isobutanebp -10.2°C



cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24

0Isomers of Hexane

9

Constitutional Isomers of Hexane C6H14



cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24

0Isomers of Pentane

10

• Pentane - C5H12



cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24

0Alkane Isomers

11



cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24

0Alkyl Groups

12

H3C Cl OHmethyl chloride ethyl alcohol

ClCl

ClCl

n-butyl chloride sec-butyl chloride iso-butyl chloride tert-butyl chloride

1-chlorobutane 2-chlorobutane 1-chloro-2-methylpropane 2-chloro-2-methylpropane



cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24

0Degree of Alkyl Substitution

13

primary carbon1°

C

H

R

H

H

R = any carbon alkyl group

C

H

R

R

H

C

H

R

R

R C

R

R

R

R

secondary carbon2°

tertiary carbon3°

quaternary carbon4°

• We designate a kind of carbon (or kind of functional group attached to that carbon) according to how many other alkyl groups are attached to it.



cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24

0 Naming Alkanes



cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24

0Naming Alkanes

15

• With all these isomers possible, how can we distinguish them?

• We need to have a systematic method to name all these isomers



cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24

0IUPAC Nomenclature for Linear Alkanes

16

• International Union of Pure and Applied Chemistry



cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24

0IUPAC Nomenclature for Branched Alkanes

17

Prefix - Parent - Locant - Suffix

where and whatare the substituents

longest carbonchain

Where is the primaryfunctional group

what is the primaryfunctional group



cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24


18

• Substituted derivatives of the unbranched alkane

• STEP 1 - Find the longest continuous chain of carbons. If there is more than one possibility, choose the chain that is more branched. This is reference to as the PARENT chain.

• STEP 2 - Identify the substituent groups attached to the parent chain (-ane ending changed to -yl)

• STEP 3 - Number the chain beginning at the end of the nearest substituent. If there are substituents equal distance from either end, look for the next nearest branch.



cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24


19

• STEP 4 - Write the name for the molecule using hyphens between prefixes and commas between numbers.

• If there are more than one substituent on the same carbon, they would each have the same number.

• If there is more than one substituent with the same name, indicate the number of them using di, tri, tetra, etc.

• Prefixes are arranged in alphabetical order according to the substituent name.



cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24


20

Constitutional Isomers of Hexane C6H14



cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24


21

identify longest chain

identify the substituents

methyl

methyl

ethyl

12

3 45

6

78

3-methyl

4-methyl

6-ethyl

Number the chain

6-ethyl-3,4-dimethyloctane



cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24

0

Properties and Conformations of Alkanes



cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24

0Boiling Point of Alkanes

23

Alkanes CnH2n+2

MF StructureName

Methane CH4CH4

Ethane C2H6

Propane

Butane

Pentane

Hexane

Heptane

Octane

Nonane

Decane

CH3CH3

C3H8

C4H10

C10H22

C5H12

C6H14

C7H16

C8H18

C9H20

bp

-164°C

-89°C

-42°C

-0.5°C

174°C

36°C

69°C

98°C

125°C151°C



cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24

0Boiling Point of Branched

24

• Branching decreases van der Waals attractions resulting in a decrease in the boiling point

Pentane36°C

2-methylbutane28°C

2,2-dimethylpropane10°C



cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24

0Reactions of Alkanes

25

• Alkanes are relatively inert

• will only react with very reactive species

CH4 + O2 CO2 + H2O + HEATCombustion

Free Radical Chlorination

CH4 + Cl2 CH3Cl + +CH2Cl2 CHCl3 + CCl4



cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24

0Heat of Combustion

26

• The heat realeased on combustion is the Heat of Combustion

CH4 + O2 CO2 + H2O2 2

ΔH° = -890 kJ/mol (-212.8 kcal/mol)

ΔH° = ΔH°products - ΔH°reactants

+ O2 CO2 + H2O8 6

ΔH° = -3529 kJ/mol (-843.4 kcal/mol)

5



cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24

0Relative Stability

27

• Comparing the heat of combustion (potential energy in the molecule) of isomers can tell us about relative stability



cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24

0Conformations

• All single bonds freely rotate at room temperature (unless constrained by a ring).

• Thus, linear alkanes are in constant motion.

• If the molecules were frozen to absolute zero you could see different arrangements of the groups depending on the state of the bond rotations.

• Conformers: Different rotational isomers (conformations) of a molecule.

• Conformational Analysis: Study of how conformations affect a molecule.

28



cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24

0Representation of 3D Structures

• Sawhorse Projection: A view of a molecule showing wedges and dashes for bonds coming out or going into the plane of the paper - resembles a sawhorse.

29



cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24

0Representation of 3D Structures

• Newman Projection: A view of a molecule looking straight down one C-C single bond (see below).

30



cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24

0Ethane Conformations

31

C CH

H H

HH

H

Ethane Conformations

sawhorse

view down this axis to see Newman Projection

H

H

H

H

H

H

Newman Projectionstaggered

60° rotate front C

by 60° H

H

HHH

HNewman Projection

eclipsed

0°

higher in energy by 2.9 kcal/mol

E

rotation0° 60° 120° 180° 240° 300° 360°

Staggered Staggered Staggered Staggered

Eclipsed Eclipsed Eclipsed

2.9 kcal/mol



cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24

0A few more terms

• Staggered Conformation: one in which the relationship of the groups on one carbon versus an adjacent carbon (front and back on a Newman projection) are aligned 60° apart.

• Eclipsed Conformation: one in which the relationship of the groups on one carbon versus an adjacent carbon are aligned 0° apart.

32

H

H

H

H

H

H


60°

H

H

HHH

HNewman Projection

eclipsed

0°



cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24

0A few more terms

• Torsional Strain: The strain introduced by electron repulsion of bonds on adjacent carbons. This is highest when the bonds are eclipsed and lowest when staggered.

• Steric Strain: The strain introduced when atoms are forced to become close to each other (they bump into each other).

33

H

H

H

H

H

H


60°

H

H

HHH

HNewman Projection

eclipsed

0°



cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24

0

34

anti Butane Conformations

CCH3C

CH3H

HH

H

Butane Conformations

sawhorse

view down this axis to see Newman Projection

H

CH3

H

H

CH3

H

Newman Projectionanti-staggered

anti-methyls - 180°



cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24

0anti-Butane

35



cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24

0

36

eclipsed Butane ConformationsButane Conformations

H

CH3

H

H

CH3

H

Newman Projectionanti-staggered

rotate front Cby 60°

H

CH3

HH3CH

HCH3 and H

higher in energy by 3.8 kcal/mol

CH3 and H

Newman Projectioneclipsed



cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24

0Eclipsed Butane

37



cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24

0

38

gauche Butane Conformations

Butane Conformations

H

CH3

H

H3C

H

H

Newman Projectiongauche-staggered


H

CH3

HH3CH

H 0.9 kcal/mol higher in energy than the anti-staggered (lowest energy) conformation


60°



cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24

0Gauche Butane

39



cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24

0

40

eclipsed Butane ConformationsButane Conformations

H

CH3

H

H3C

H

H

Newman Projectiongauche-staggered


CH3 and CH3

4.5 kcal/mol higher in energy than the anti-staggered conformationH

CH3

HH H

CH3




cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24

0syn-eclipsed Butane

41



cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24

0Butane Energy Profile

42

E

rotation0° 60° 120° 180° 240° 300° 360°

Staggeredanti

Staggeredgauche

Staggeredgauche

Staggeredanti

Eclipsed

EclipsedMethyls Aligned

Eclipsed

3.8 kcal/mol 4.5 kcal/mol 0.9 kcal/mol

H

CH3

H

H

CH3

H

H

CH3

HH3CH

H

H

CH3

H

H3C

H

H

60°

H

CH3

HH H

CH3

H

CH3

H

H3C

H

H

60°

H

CH3

HH3CH

H

H

CH3

H

H

CH3

H



cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24

0Higher Alkanes

• The most stable conformation in longer chain alkanes is the all-anti conformation

43

CC

CC

CCH

HH

H

H

H

H

H

H

H

H

H

H

H



cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24

0 Cycloalkanes



cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24

0Cycloalkanes

45

• Alkanes can be connected in a circle

cyclopropane

H2C

CH2

CH2 H2C

H2C CH2

CH2CH2H2C

H2CCH2

CH2

CH2H2C

H2C

H2C CH2

CH2

cyclobutane cyclopentane cyclohexane

methylcyclopentane1,3-dimethylcyclohexane

3-ethyl-1,1-dimethylcyclohexane



cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24

0cis and trans Cycloalkanes

46

Br Br

Br

H

Br

Hcis-1,2-dibromocyclopropane

Br

H

H

Br

trans-1,2-dibromocyclopropane



cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24

0A few more terms

• Stereoisomers: Isomers (different compounds) that have all the same number and kind of atoms that are all connected the same, but differ in their arrangement in three dimensions.

• Due to the restricted rotation in cycloalkanes, molecules with more than one substituent could have the groups either on the same side (cis) or opposite sides (trans) of the plane of the ring. These are stereoisomers.

47



cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24

0cis and trans dimethylcyclopentane

48

=

cis-1,3-dimethylcyclopentane

H3C

H

CH3

HH3C CH3

=

trans-1,3-dimethylcyclopentane

H3C

H

H

CH3H3C CH3



cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24

0Cyclic Compounds

• Ring Strain

• Angle Strain: the strain due to bond angles being forced to expand or contract from their ideal.

• Torsional Strain: the strain due to electron repulsion of eclipsing bonds.

• Steric Strain: the strain due to atoms coming too close.

49



cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24

0Cyclic Compounds

• Heat of Combustion: the amount of heat (energy) released when a molecule burns completely with oxygen.

50

3 4 5 6 7 8 9 10 11 12 13 14Ring Size

Rin

g St

rain



cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24

0Cyclopropane

• Highest amount of angle strain

51

60°



cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24

0Cyclopropane

52



cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24

0Cyclobutane

53



cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24

0Cyclopentane

54



cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24

0Cyclohexane

55



cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24

0Axial and Equatorial Positions of

Cyclohexane

56

Pink - AxialBlue - Equatorial



cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24

0Ring Flips in Cyclohexane

57



cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24

0Energy of Chair Flip Conformations

58



cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24

0Cyclohexane Axial Positions More

Crowded

59

H

CH3

H

HH

HH

CH3

H

H

H

Haxial methyl equatorial methyl

1,3-diaxial interaction

1.8 kcal/mol more stable conformation

ring flip



cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24

0axial Methyl Cyclohexane

60



cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24

0equatorial Methyl Cyclohexane

61



cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24

0methylcyclohexane Ring Flips

62



cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24

0dimethylcyclohexane

63

CH3

HCH3

H

ax

eq

cis-1,2-dimethylcyclohexane

same interactions in both conformations -- equal in energy

ring flip

CH3

H

CH3

Heq

ax

ring flip

1

21

221

CH3

CH3

CH3

HCH3

H

ax

eq

trans-1,2-dimethylcyclohexane

different interactions in both conformations -- NOT equal in energy

H

CH3

H

CH3eqax

1

21

221

CH3

CH3lower in energy



cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24

0cis-1,2-dimethylcyclohexane

64



cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24


65


http://cook.chem.ndsu.nodak.edu/chem341/chem341_04/images/rings/1-2-cis-dime-cyclohexane-top-sphere.jpg


cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24

0trans-1,2-dimethylcyclohexane

66



cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24


67



cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24


68



cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24


69

CH3

HCH3

H

ax

eq

cis-1,3-dimethylcyclohexane

same interactions in both conformations -- equal in energy

ring flip

axring flip1 1

3

1CH3

CH3different interactions in both conformations -- NOT equal in energy

lower in energy

3

CH3

H

H3C

H

3eq

CH3

HCH3

H

ax

eq

trans-1,3-dimethylcyclohexane

ax

1 1

3

1CH3

CH3

3

H

H3C

H

CH3

3

eq



cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24

0Boat Conformations

70

boat cyclohexane

H

H

H

HH H

H H

norbornane



cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24

0

71

Sugar Structure

• Glucose

OHHHHOOHHOHH

CH2OH

OH

HO

H

HO

H

HOHH OH

OH

OH



cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24

0

72

Polysaccharides

• Starch



cook

.che

m.n

dsu.

noda

k.ed

u/ch

em24

0Polycyclic Molecules

73

O

OR

R

CO2R

RH

H

H HH H

fused spiro bridged


chapter 03 alkanes and cycloalkanes -...

Documents