D.M. Collard 2007

TOPIC 3. ALKANES (chapter 4 and parts of chapters 7

and 10)

L

OBJECTIVES

1. Develop rules for systematic nomenclature of alkanes alkenes and alkynes2. Describe the geometry of hydrocarbons3. Describe the rotation around C-C single bonds (conformation)4. Describe the geometry of cyclic alkanes

D.M. Collard 2007

SYSTEMATIC IUPAC NOMENCLATURE OF ALKANES

Linear (Unbranched) AlkanesCH4

CH3CH3

CH3CH2CH3

CH3(CH2)2CH3

CH3(CH2)3CH3

CH3(CH2)4CH3

CH3(CH2)5CH3

CH3(CH2)6CH3

CH3(CH2)7CH3

CH3(CH2)8CH3

Prob4.37

C1

2

3

4

5

6

7

8

9

10

11

12

13

14

15 pentadecane

16 hexadecane

17 heptadecane

18 octadecane

19 nondecane

20 eicosane

methane

ethane

propane

butane

pentane

S:4.1-2

Branched alkanese.g.,

find longest continuous chain of carbonsbase name = alkane

List substituents as prefixes in alphabetical order (ignoring di, tri, sec, tert; but do not ignore “iso”)

octane

D.M. Collard 2007

Number longest continuous chain from the end that places a substituent at the lowest possible number

ethyl dimethyloctane

e.g.,

Problem: Name the following.

D.M. Collard 2007

Problem: Isooctane is used as a standard to measure the rate of gas combustion. Isooctane, C8H18, has five 1° carbons, one 2° carbon, one 3°carbon and one 4° carbon. Three of the methyl groups are attached to a 4°carbon atom, and two methyl groups are attached to a 3° carbon. What is the IUPAC systematic name of isooctane?

Problem: A female tiger moth pheromone, C18H38, has three 1° carbons, fourteen 2° carbons and one 3° carbon. Two of the methyl groups are attached to a 3° carbon atom, and one methyl group is attached to a 2°carbon. What are the IUPAC names of the compounds which fit this description?

D.M. Collard 2007

Alkyl Halidese.g.

Alcoholse.g.

OH

Cl

Named as alkanolslongest chainnumbered to put –OH in lowest possible positionsubstitutents named and positions given

Named as haloalkaneslongest chainsubstitutents (incl. halogen) namedLowest possible position of substituent defines positions

NAMING OTHER COMPOUNDSS:4.3

Cyclic AlkanesNamed as (substituted-)cycloalkanescompare formulas:

alkanes (CnH2n+2) with cyclic alkanes (CnH2n - one ring)

Prob:4.22,27,33,

38,39,41

S:4.4

D.M. Collard 2007

Problem: Draw all of the constitutional isomers of monocyclic alkanes with the molecular formula C6H12. [There are twelve possible structures]

Bicyclic AlkanesNamed as bicyclo[x,y,z]alkanes

Problem. Name the following bicycliccompounds.

S:4.15

D.M. Collard 2007

Alkenese.g.,

Find longest continuous chain of carbons which contains the C=C (or C≡C):base name = alkene (alkyne)

Number longest continuous chain of carbons to assign an sp2 (sp) carbon to as low a number as possible: #-alkene (#-alkyne)

heptene

Identify substituents and position of substitutents. Substituents are listed alphabetically

3-heptene

LProb: 4.25

S:4.5-6

Cyclic AlkenesDefine the sp2 carbons of the alkene as C-1 and C-2 such that the first encountered substituent occupies the lowest possible number (this is sometimes called the first point of difference rule).

What about compounds with alcohol and double (or triple) bonds? Consider them alcohols first, and name them as (substituted alkanols)

Cl

OH

OH

D.M. Collard 2007

Alkanes : CnH2n

Alkenes: CnH2n Cyclic alkanes : CnH2n

Dialkenes: CnH2n-2 Alkynes: CnH2n-2

For each ring or pi-bond in a molecule there are two fewer hydrogen atoms than expected for a non-cyclic alkane, so:

Number of rings = (2#C + 2 – #H) / 2

Problems: How many pi bonds or rings are there in C9H14?

C10H17?

MOLECULAR FORMULAS:4.18

The same equation is true in the presence of divalent oxygen or sulfur atoms:

For C,H,O,S: SODAR = (2#C + 2 – #H) / 2

In the presence of halogens (monovalent) or nitrogen (trivalent):

For C,H,O,N,S, Hal: SODAR = (2#C + 2– #H – #Hal + #N) / 2

(each Hal replaces a H; each N adds an extra H; and S,O have no effect on the calculation)

D.M. Collard 2007

Problem: Determine the value of SODAR for compounds with the following molecular formulas.

SODAR

C6H6

CH3Br

CH5N

C3H8N

Differentiating alkenes and cycloalkanes

Catalytic hydrogenation

Problem. Compound A, C4H6, undergoes hydrogenation to afford C4H10. (a) How many rings are present in A? (b) How many pi bonds? (c) Draw all of the possible structures of A.

H2/Pt

H2/Pt

D.M. Collard 2007

Problem: α-Pinene, C10H16, found in cedar wood oil and a major constituent of oil of turpentine, undergoes catalytic hydrogenation to give a compound with formula C10H18. How many double bonds does α-pinene have? How many rings?

Problem: Zingiberene, C15H24, isolated from ginger root, undergoes catalytic hydrogenation to give a compound with formula C15H30. How many double bonds does zingiberene have? How many rings?

D.M. Collard 2007

PHYSICAL PROPERTIES OF ALKANES

C1-C4: gas C5-C15: liquid >C16: solid large n: polyethylene

Branching usually decreases m.p.: - branches impede crystal packing.

(Highly symmetric molecules have higher m.p.e.g., C(CH3)4, m.p.=-16 °C; CH3(CH2)3CH3, m.p.=-130 °C)

Branching decreases b.p.: - More compact structure gives less surface area: weaker van der

Waals interactionsNon-polar: immiscible with waterLess dense than water Dissolve other non-polar organic molecules

Flammable!Chemically inertUses: fuels, solvents,waxes

S:4.7

CONFORMATIONAL ANALYSIS: ROTATION AROUND C-C SINGLE BONDS

Rotation around C-C single bonds leads to formation of different conformers. The energy required to interconvert conformers is small: Conformers can rarely be isolated at room temperature.

H

HH

HH

HH

H

H

HH H

NewnamProjections

Prob:4.36

S:4.8-9

D.M. Collard 2007

B

C

A BA

The eclipsing conformation of ethane is destabilized by torsional strain.

The eclipsing conformations of butane are further destabilized by steric strain

H

H

H

HHH

H3C

H

CH3

HHH

D.M. Collard 2007

Ethane

H

H HH

HHH

H

H

H

HHH

H HH

HHH

H HH

HHH

H

H

H

HHH

H

H

H

HHH

H

H

H

HH

Dihedral angle:Φ=

360(0)0 60 120 180 240 300

Ene

rgy

/ kJ.

mol

-1

Φ - °

0

12

Butane

360(0)

Dihedral angle:Φ=

0 60 120 180 240 300

Ene

rgy

/ kJ.

mol

-1

Φ - °

H

H MeMe

HHH

Me

H

Me

HHMe

H HMe

HHH

Me HMe

HHH

H

Me

Me

HHMe

H

H

Me

HH H

Me

H

Me

HH

0

19

16

4

D.M. Collard 2007

RELATIVE STABILITY OF CYCLOALKANES

Ring strain: Heat of combustion per methylene unit in cycloalkanen ΔHc/n - kJ/mol/carbon3 -6974 -6865 -6646 -6597 -662

(CH2)n + 3n/2 O2 n CO2 + n H2OΔHc

L S:4.10

CONFORMATIONAL ANALYSIS OF SUBSTITUTED CYCLOHEXANES

The Shape of Cyclohexane

S:4.11-4.15

D.M. Collard 2007

cis and trans Stereoisomers

Problem: Draw chair conformations of the following

D.M. Collard 2007

Ring Inversion (“Ring Flip”)

Conformational Preferences of Substituted Cyclohexanes

CH3

HH

H

H

H

H

H

H

H

H

H

H

HH

H

H

H

H

CH3

H

H

H

H

7.5 kJ mol-1more stable

D.M. Collard 2007

Conformational Preferences of DisubstitutedCyclohexanes:

1,2-Dimethylcyclohexanes

1,3-Dimethylcyclohexanes

Problem: Determine the preferred conformation of the cis and trans 1,4-dimethylcyclohexane [Draw chair conformations to decide]. How could you experimentally demonstrate this difference in stability?

D.M. Collard 2007

Conformational Preferences of Disubstituted CyclohexanesBearing Different Substituent

e.g.,

Larger substituents have a stronger preference for the equatorial position

FREE-RADICAL HALOGENATION OF ALKANES

Overall Reaction

Alkane + Halogen Alkyl Halide + Hydrogen Halide

Reaction is selective for substitution of hydrogen atoms on 3° over those on 1°and 2° positions.e.g.,

Bromination is more selective than chlorination.

Br2hν

R-H + F2

R-H + I2

BOOM!

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S:4.16, 10.2-10.6

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D.M. Collard 2007

MechanismStep 1: Initiation - generation of radicals

Br Br

H RBr

hν

Step 2: Propagation - formation of product and regeneration of radicals

Br Br R

D.M. Collard 2007

Step 3: Termination - consumption of radicals

CC

C Br

BrBr

Regioselectivity

Br2hν

Br

Br

Br

Br

Br

D.M. Collard 2007

Radical bromination of alkanes is only useful when replacing a hydrogen on a 3o carbon. Otherwise the reaction is unselective... e.g.,

…or when there is only one possible monobrominated product.e.g.,

Br2hν

Br2hν

Br2hν

Origin of Regioselectivity

Bond Dissociation Energies (R-H → R. + H.)

kJ/mol 435 410 395 381

Radical (R.) stability

< < <

CH3CH2 H (CH3)2CH H (CH3)3C HCH3 H

CH3CH2 (CH3)2CH (CH3)3CCH3

D.M. Collard 2007

Energetics

Fluorination(a) F. is very reactive and reacts unselectively with 1°, 2° and 3° C-H (b) R. + F2 → R-F + F. Is very, very exothermic (uncontrolable!)

ChlorinationCl. is quite reactive and provides some selectivity in reaction with 1°, 2° and 3° C-H highly exothermic

BrominationBr. is less reactive than Cl. and provides more selectivity in reaction with 1°, 2° and 3° C-H exothermic

IodinationI. is unreactive and reacts very slowly with C-H; I. + R3C-H → H-I + R3C. endothermic

BOOM!

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SYNTHESES OF ALKANES

Hydrogenation of Alkenes and Alkynes

Hydrogenation is stereospecific

H2

Pt or Pd

H2

Pt or PdC C

C C

CH3

CH3

H2

Pt or Pd

S:4.17

D.M. Collard 2007

TOPIC 3 ON EXAM 2

Types of Questions- Describe structure and bonding in alkanes, alkenes, alkynes and

cycloalkenes.- Provide systematic names for hydrocarbons- Analyze conformational preferences of alkanes and substituted

cyclohexanes.- Describe reactions of alkenes.The problems in the book are good examples of the types of problems on

the exam.

Preparing for Exam 2- Work as many problems as possible. - Work in groups.- Do the “Learning Group Problem” at the end of the chapter.- Work through the practice exam