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Chem 207 B. R. Kaafarani 1

Chapter 2Hydrocarbon Frameworks:

Alkanes

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Hydrocarbons

Aromatic

Aromatic Aliphatic

Aliphatic

Alkanes

Alkanes Alkynes

Alkynes Alkenes

Alkenes

C C

H

H

H

H

H

H C C

H

HH

H

C C HH

H H

H

HH

H

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1. Valence Bond Theory:

Constructive interference between electron

waves of two half-filled atomic orbitals is basis of

shared-electron bond.

2. Molecular Orbital Theory:

Derive wave functions of molecules by combining

wave functions of atoms.

Models for Chemical Bonding

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2.5. Introduction to Alkanes (CnH2n+2). The simplest

Alkanes: Methane, Ethane, and Propane

bp -160°C bp -89°C bp -42°C

Methane (CH4) CH4

Ethane (C2H6) CH3CH3

Propane (C3H8) CH3CH2CH3

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Structure of Methane

2.6. sp3 Hybridization and Bonding in

Methane

Tetrahedral

Bond angles = 109.5°

Bond distances = 110 pm

But structure seems inconsistent with

electron configuration of carbon.

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Electron configuration of carbon

2s

2p

Only two unpaired electrons.

Should form bonds to only two

hydrogen atoms.

Bonds should be at right angles

to one another.

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2s

2p

sp3 Orbital Hybridization

Promote an electron from the 2s to the 2p orbital.

2p

2s

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2p

2s


Mix together (hybridize)the 2s orbital and the

three 2p orbitals.

2 sp3

4 equivalent half-filled

orbitals are consistentwith four bonds and

tetrahedral geometry.

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Nodal Properties of Orbitals

s

p + –

+

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Shape of sp3 hybrid orbitals

p + – s +

Take the s orbital and

place it on top of the p orbital.

s + p + – +

Reinforcement of electron wave

in regions where sign is the same.

Destructive interference in

regions of opposite sign.

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Shape of sp3 hybrid orbitals

sp hybrid + –

Orbital shown is sp hybrid.

Analogous procedure using one s orbital and three p orbitals

gives sp3 hybrid.

Shape of sp3 hybrid is similar.

Hybrid orbital is not symmetrical.

Higher probability of finding an electron on one side of the

nucleus than the other.

Leads to stronger bonds.

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–

+ –

The C—H Bond in Methane

sp3s CH

H—C CH

gives a bond.

In-phase overlap of a half-filled 1s orbital of hydrogen

with a half-filled sp3 hybrid orbital of carbon:

+

+

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Justification for Orbital Hybridization

Consistent with structure of methane.

Allows for formation of 4 bonds rather than 2.

Bonds involving sp3 hybrid orbitals are stronger

than those involving s-s overlap or p-p overlap.

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Structure of Ethane

CH3CH3

C2H6

2.7. Bonding in Ethane

Tetrahedral geometry at each carbon.

C—H bond distance = 110 pm.

C—C bond distance = 153 pm.

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The C—C Bond in Ethane

In-phase overlap of half-filled sp3 hybrid orbital of one

carbon with half-filled sp3 hybrid orbital of another.

Overlap is along internuclear axis to give a

bond.

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Chem 207 B. R. Kaafarani 17bp -0.4

°C bp -10.2

°C

n-Butane CH3CH2CH2CH3

Isobutane (CH3)3CH

2.8. Isomeric Alkanes: The Butanes

C4H10

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CH3CH2CH2CH2CH2CH3

n-Hexane

n-Pentane

CH3CH2CH2CH2CH3

CH3CH2CH2CH2CH2CH2CH3

n-Heptane

2.9. Higher n-Alkanes

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n-Pentane

CH3CH2CH2CH2CH3

Isopentane

(CH3)2CHCH2CH3

Neopentane

(CH3)4C

2.10. The C5H12 Isomers

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Table 2.1 Number of Constitutionally Isomeric Alkanes

CH4 1 C8H18 18

C2H6 1 C9H20 35

C3H8 1 C10H22 75

C4H10 2 C15H32 4,347C5H12 3 C20H42 366,319

C6H14 5 C40H82 62,491,178,805,831

C7H16 9

How many isomers?

The number of isomeric alkanes increases as the number

of carbons increases.

There is no simple way to predict how many isomers there

are for a particular molecular formula.

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IUPAC (International Union of Pure and Applied

Chemistry) NomenclatureA. Parent chains: normal alkanes

Parent names:

CH4 methane n-C11H24 undecane

CH3CH3 ethane n-C12H26 dodecane

CH3CH2CH3 propane n-C13H28 tridecane

CH3(CH2)2CH3 butane n-C14H30 tetradecane

CH3(CH2)3CH3 pentane ¦

CH3(CH2)4CH3 hexane n

-C20H42 icosanen-C7H16 heptane n-C30H62 triacontane

n-C8H18 octane n-C40H82 tetracontane

n-C9H20 nonane ¦

n-C10H22 decane etc.

Systematic name: {side groups}parent chain{suffix}

know

to here

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2.11. IUPAC Nomenclature of Unbranched Alkanes

Note:

n-prefix is not part of IUPAC name of any alkane.

For example: n-butane is "common name"

for CH3CH2CH2CH3; butane is "IUPAC name."

Others:

Latin or Greek prefix for number of carbons + ane

suffix.

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B. Substituents: Alkyl groups

CH3

CH3 CH2 CH2

CH3CH2CH2CH2

CH3 CH CH2

CH3

CH3 CH2

CH3 CH CH3

CH3CH

2CHCH

3

CH3 C CH3

CH3

methyl group ethyl group

propyl isopropyl

butyl sec-butyl

isobutyl tert -butyl

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Naming Alkyl Groups

Step 1: Identify longest continuous chain starting at

point of attachment.

Step 2: Drop -ane ending from name of unbranched

alkane having same number of carbons as longest

continuous chain and replace by -yl.Step 3: Identify substituents on longest continuous

chain.

Step 4: Chain is always numbered starting at point

of attachment.

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CH3CH2CH2C CH

H H

H H

C

H

H

or

and

CH3CHCH3C CH

H H

H

HC

H

H

or

IUPAC name: Propyl

Common name: n-Propyl

IUPAC name: 1-Methylethyl

Common name: Isopropyl

The C3H7 Alkyl Groups

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CH3CH2CH2

Classification: Primary alkyl group.

Alkyl groups are classified according to the degree of

substitution at the carbon that bears the point of attachment.

A carbon that is directly attached to one other carbon is

a primary carbon. A carbon that is directly attached to two other carbons is

a secondary carbon.

CH3CHCH3* *


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IUPAC name: 1-Methylpropyl

Common name: sec-Butyl

Classification: Secondary alkyl group

CH3CHCH2CH3C CH

H H

H

HC C

H H

H H

or 1

2 3


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IUPAC name: 2-Methylpropyl

Common name: IsobutylClassification: Primary alkyl group

123C

H

CH2

CH3

CH3


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1 2

C CH3

CH3

CH3


IUPAC name: 1,1-Dimethylethyl

Common name: tert-Butyl

Classification: Tertiary alkyl group

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C. IUPAC Rules

1. Find the longest straight chain = parent chain.

2. Number the parent chain in the direction that gives the lowest

number to the substituents at the first point of difference.

3. Two or more identical groups are indicated by di, tri, tetra, etc.

CH3 CH2 CH2 CH2 CH

CH2 CH3

CH3

Longest chain is 7 (not 6).

This is 3-Methylheptane..

2,3,6-Trimethylheptane (not

2,5,6-trimethylheptane)

2,2,6,6,7-Pentamethyloctane (not

2,3,3,7,7-Pentamethyloctane)

12

34567

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2,2,6,6,7-Pentamethyloctane?

2,3,3,7,7-Pentamethyloctane?

What is correct name?

1

2 34

56

78

8

7 65

43

21

The chain is numbered in the direction that gives the lower locant to the substituent at the first point of difference in the

names.

First Point of Difference Rule

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4. Different groups are listed in alphabetical order (di, tri,

tetra, etc. don’t count; n, sec, tert don’t count; iso does).

5. If the numbering is the same in both directions, choosethe numbering to follow the alphabetical order.

6-Ethyl-5-isopropyl-2,2-dimethyloctane

5-Ethyl-6-propyldecane

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6. When two or more chains compete for the longest, the choice

goes to the one with the greater number of side groups.

7. Complex side groups (usually > 4 carbons) are named

systematically

a. Start numbering at the carbon attached to the parent chain.

b. Enclose name in parentheses.

2,4,6-Trimethyl-5-propyloctane

4-(1-Methylethyl)-5-(1,2-dimethylpropyl)

decane12

3

1

2

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Multiple complex

functional groups

are denoted by

brackets and amultiplicity denoted

not by di, tri, tetra,

etc, but by bis, tris,

tetrakis, pentakis,hexakis, etc. These

are alphabetized in

the name.

5,6-Bis(1,2-dimethylpropyl)dodecane

7,8-Bis(1,2-dimethylpropyl)-3,3,10-trimethyldodecane

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B. R. Kaafarani 35Chem 211 B. R. Kaafarani 35

What is the IUPACname of the following

compound?

3 ‐ E t h y l ‐ 4

‐ ( 1 , 2 . . .

3 ‐ E

t h y l ‐ 5 ‐ m e t h . . .

3 ‐ E

t h y l ‐ 5 ‐ m e t h . . .

4 ‐ ( 1 ‐ E t h y l p r o p . . .

2 , 3 , 5 ‐ t r i m e t h y . . .

N o n e

o f t h e

a b . . .

0% 0% 0%0%0%0%

1.3-Ethyl-4-(1,2-dimethylpropyl)-5-

methylundecane

2.3-Ethyl-5-methyl-4-pentylundecane

3.3-Ethyl-5-methyl-4-(1,2-

dimethylpropyl)undecane

4.4-(1-Ethylpropyl)-2,3,5-trimethylundecane

5.2,3,5-trimethyl-4-pentylundecane

6.None of the above Countdown3

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Cyclopentane Cyclohexane

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.

2.15. Cycloalkane (CnH2n) Nomenclature

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Ethylcyclopentane

CH2CH3

Name any alkyl groups on the ring in the usual way.

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-dimethylcyclohexane

CH2CH3

H3C CH3

Cycloalkanes

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When the ring contains fewer carbon atoms than

an alkyl group attached to it, the compound is

named as an alkane, and the ring is treated as a

cycloalkyl substituent.

3-Cyclopentyl-5-methyloctane

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What is the IUPAC parent

name of the followingCompound?

C y c l o

p r o p a n e

C y c l o

p e n t a n e

H e p t a n e

H e x a n e

0% 0%0%0%

1. Cyclopropane

2. Cyclopentane

3. Heptane

4. Hexane

Countdown

3

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What is the IUPAC name of the following compound?

1 , 1 , 2 , 3 ‐ T e t r a c . . .

1 , 2 , 4 , 4 ‐ T e t r a c . . .

1 , 1 , 2 , 4 ‐ T e t r a c . . .

1 , 2 , 4 , 4 ‐ T e t r a c . . .

25% 25%25%25%

Countdown

3

1. 1,1,2,3-

Tetracyclopropylpropane

2. 1,2,4,4-Tetracyclopropylpropane

3. 1,1,2,4-Tetracyclopropylbutane

4. 1,2,4,4-Tetracyclopropylbutane

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Boiling Points of Alkanes

2.17. Physical Properties of Alkanes and

Cycloalkanes

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.

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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).

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+ – + –

+ – + –

The result is a small attractive force between the two molecules.

Temporary dipole in one molecule (left) induces a complementary

dipole in other molecule (right).

Induced dipole-Induced dipole attractive forces

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Boiling Points

Increase with increasing number of carbons:

- More atoms, more electrons, more

opportunities for induced dipole-induceddipole forces.

Heptane

bp 98°C

Octane

bp 125°C

Nonane

bp 150°C

Boiling Points

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Boiling Points

Decrease with chain branching:

- Branched molecules are more compact

with smaller surface area— fewer points of contact with other molecules.

Octane: bp 125°C

2-Methylheptane: bp 118°C

2,2,3,3-Tetramethylbutane: bp 107°C

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All alkanes burn in air to give

carbon dioxide and water.

2.18Chemical Properties:

Combustion of Alkanes

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Heats of Combustion

Increase with increasing number of carbons:

- More moles of O2 consumed, more moles

of CO2 and H2O formed.

Decrease with chain branching:

- Branched molecules are more stable (have

less potential energy) than their unbranched

isomers.

f C

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4817 kJ/mol

5471 kJ/mol

6125 kJ/mol

654 kJ/mol

654 kJ/mol

Heptane

Octane

Nonane

Heats of Combustion

H t f C b ti

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5471 kJ/mol

5466 kJ/mol

5458 kJ/mol

5452 kJ/mol

5 kJ/mol

8 kJ/mol

6 kJ/mol

Heats of Combustion

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Important Point

Isomers can differ in respect to their stability.

Equivalent statement:Isomers differ in respect to their potential energy.

Differences in potential energy can be measuredby comparing heats of combustion.

Figure 2 17

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8CO2 + 9H2O

5452 kJ/mol5458 kJ/mol

5471 kJ/mol

5466 kJ/mol

O2+25

2

Figure 2.17

O2+25

2 O2+25

2

O2+ 252

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2.19. Oxidation-Reduction inOrganic Chemistry

Oxidation of carbon corresponds to an increasein the number of bonds between carbon and

oxygen and/or a decrease in the number of

carbon-hydrogen bonds.

O

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Increasing oxidation

state of carbon

-4 -2 0 +2 +4

H

H

H

C H

H

H

H

C OH

O

C HH

O

COHH

O

COHHO

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Increasing oxidation

state of carbon

-3 -2 -1

HC CH

C C

H

H H

H

C C H

HH

H H

H

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Fortunately, we rarely need to calculate the

oxidation state of individual carbons in a molecule .

We often have to decide whether a process is an

oxidation or a reduction.

Generalization

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X Yoxidation

reduction

C C

Generalization

X less electronegative than carbon.

Y more electronegative than carbon.

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 reverseprocess is reduction.

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CH3Cl HClCH4 Cl2+ +Oxidation

+ 2Li LiClCH3Cl CH3Li +

Reduction

Examples

2.20. sp2 Hybridization and Bonding in Ethylene

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C2H4

H2C=CH2

Structure of Ethylene

- Planar

- Bond angles: close to 120°

- Bond distances: C—H = 110 pm

C=C = 134 pm


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2s

2p

p y

2p

2s

Mix together (hybridize)

the 2s orbital and two of

the three 2p orbitals

Promote an electron from

the 2s to the 2p orbital.


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2p

2s

p y

2 sp2

3 equivalent half-filled sp2

hybrid orbitals plus 1 p

orbital left unhybridized.


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y

sp2 Orbital H bridi ation

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2 sp2

p

Bonding in Ethylene

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-Bonding in Ethylene

2 sp2

The unhybridized p orbital of

carbon is involved in

bonding to the other carbon.

p

B di i Eth lB di i Eth l

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Bonding in Ethylene Bonding in EthyleneBonding in Ethylene

22 spsp22

pp

each carbon has an unhybridized 2each carbon has an unhybridized 2pp orbitalorbital

axis of orbital is perpendicular to the plane of theaxis of orbital is perpendicular to the plane of the bondsbonds


B di i Eth lBonding in Ethylene

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Bonding in Ethylene Bonding in EthyleneBonding in Ethylene

22 spsp22

pp

side-by-side overlap of half-filledside-by-side overlap of half-filled

pp orbitals gives aorbitals gives a bondbond

double bond in ethylene has adouble bond in ethylene has a

component and acomponent and a componentcomponent


2.21. sp Hybridization and Bonding in Acetylene

Structure of Acetylene

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C2H2

HC CH

Structure of Acetylene

- Linear

- Bond angles: 180°

- Bond distances: C—H = 106 pm

CC = 120 pm

sp Orbital Hybridization

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2s

2p

Promote an electronfrom the 2s to the 2p

orbital.

2p

2s

Mix together (hybridize)the 2s orbital and one of

the three 2p orbitals


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2p

2s

2 sp

2 p

2 equivalent half-filled sp

hybrid orbitals plus 2 p

orbitals left unhybridized.


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2 sp

2 p

-Bonding in Acetylene

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Bonding in Acetylene

The unhybridized p orbitals of carbon are involved in separate

bonds to the other carbon.

2 sp

2 p

Bonding in AcetyleneBonding in Acetylene

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Bonding in Acetylene Bonding in AcetyleneBonding in Acetylene

22 spsp

22 pp


Chapter2-6-Chem207

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