CH1810 Lecture #4

Organic Chemistry Basics

Carbon and The Scope of Organic Chemistry

The Position of Carbon in the Periodic TableThe Position of Carbon in the Periodic Table

Organic Chemistry - General Description

A. The Study of Carbon Compounds

B. “Organic” reminds us of plant or animal origins 1. “Natural” medicines: morphine, penicillin 2. “Natural” fibers: cotton, silk, wool 3. Foodstuffs: Fats, carbohydrates, proteins, vitamins 4. “Natural” rubber

C. Man-made substances can also be “organic” 1. Drugs: xylocaine, aspirin, acetaminophen 2. Fibers: nylon, dacron, rayon 3. Polymers: Saran wrap, polyesters, teflon, nylon 4. Synthetic rubber, synfuels

Differences Between Organic and Inorganic Compounds

Inorganic OrganicElements Present Metals and Nometals Mostly Carbon

Bonding Covalent and Ionic Mostly Covalent

“Particles” IONS & molecules ions & MOLECULES

Melting Points Relatively High Relatively LowBoiling Points Relatively High Relatively Low

Electrolytes STRONG to weak weak to NON

Comparison of Physical Properties of Organic and Inorganic Compounds

Name Salt Ethyl alcohol Benzene

Formula NaCl C2H6O C6H6

Organic/Inorganic Inorganic Organic Organic

Melting Point 804 ºC -117 ºC 5 ºC

Boiling Point 1413 ºC 78 ºC 80 ºC

Burns in O2 ? No Yes Yes

Water Soluble ? Yes Yes No

Structural Representation of Simple Alkanes

Review of Valence Bond Theory

Carbon Bonding in Alkanes

Carbon atoms in alkanes have the following characteristics:

Carbon is tetravalent: each carbon has four bonds; The four bonds possess tetrahedral geometry; The four bonds are equivalent and have similar properties.

The angle between any two bonds of a tetrahedral carbon in any carbon compound is 109.5o, the tetrahedral bond angle.

Carbon: ground-state electron configuration

2p

2s

1s

2p

2s

1s

Carbon: excited-state electron configuration

Carbon:

sp3-hybridized electron configuration

2sp3

1s

}hybridization

excitationEn

erg

yThe sp3 Hybridization Scheme for Carbon

The sp3 Hybridization Scheme

excitation

hybridization

an s and 3 p orbitals combine

4 sp3 hybrid orbitals

which are represented as

C

The Hybrid Orbital Picture of Methane

CH4

C

4 carbon sp3 hybrid orbitals

4 hydrogen 1s atomic orbitals

Methane

Ethane, CH3CH3The sp3 hybrid orbitals of

two carbons overlap

The remaining sp3 hybrid orbitals overlap with hydrogen s orbitals

7 sigma bonds

Carbon: ground-state electron configuration

2p

2s

1s

2p

2s

1s

Carbon: excited-state electron configuration

Carbon:

sp2-hybridized electron configuration

2p

2sp2

1s

}hybridization

excitation

En

erg

yThe sp2 Hybridization Scheme for Carbon

2p 2sp2

1s

An sp2-hybridized Carbon

The sp2 hybrid orbitals of two carbons overlap

The remaining sp2 hybrid orbitals overlap with hydrogen s orbitals

Ethene, CH2CH2

5 sigma bonds

The remaining unhybridized p orbitals overlap to form a ∏ bond.

∏ bond contains 2 electrons.

5 sigma bonds and 1 ∏ bond

Ethene, CH2CH2

Carbon: ground-state electron configuration

2p

2s

1s

2p

2s

1s

Carbon: excited-state electron configuration

Carbon:

sp2-hybridized electron configuration

2p

2sp2

1s

}hybridization

excitation

En

erg

yThe sp Hybridization Scheme for Carbon

2p 2sp2

1s

An sp2-hybridized Carbon

The sp hybrid orbitals of two carbons overlap

The remaining sp hybrid orbitals overlap with hydrogen s orbitals

Ethyne, CHCH

3 sigma bonds

The remaining unhybridized p orbitals overlap to form 2 ∏ bonds.

3 sigma bonds and 2 ∏ bonds

Each ∏ bond contains 2 electrons.

Bonding in HCCHH C C H

H

C

C

H

Structural Formulas and Bond Angles

120º

180º

Methane (109.5º angles)Ethane (109º angles)

CC

H

H

H

HH

H 109.5º

109.5º

“Sigma” and “Pi” Bonds

𝝈 π

7 sigma bonds

5 sigma bonds and 1 ∏ bond

3 sigma bonds and 2 ∏ bonds

C CH

HH

HH

H

C CH

H

H

H

C CH H

ethane

ethene

ethyne

Occupied “bonding” orbitals

Alkanes Aldehydes

Alkenes Ketones

Alkynes Carboxylic Acids

Aromatics Esters

Alcohols Amines

Ethers Amides

Families of Organic Compounds

C C

C C

C C

CC

CC

C

C

C O

H

C OC

CH

O

CC

O

C

CO

H

O

C

O

OC

N

CN

O

Alkanes

Hydrocarbons The Simplest Class of Organic Compounds

C. Hydrocarbons are further subclassified

C C C C C

UNSATURATED

ALIPHATIC AROMATIC

SATURATED

alkanes alkenes alkynes aromatics

Unsaturated hydrocarbons – alkenes or alkynes that can combine with H2 to form alkanes in a process called

CC

CCCC

HH

H

H H

H + 3 H2 C C C

CCC

H H

H

H

H

H

H

H

H

H

H

H

Benzene Cyclohexane

catalytic surface

catalytic surface

catalytic surface

CH4

CH3CH2CH2CH2CH2CH2CH2CH2CH2CH3

CH3CH2CH2CH2CH2CH2CH2CH2CH3

CH3CH2CH2CH2CH2CH2CH2CH3

CH3CH2CH2CH2CH2CH2CH3

CH3CH2CH2CH2CH2CH3

CH3CH2CH2CH2CH3

CH3CH2CH2CH3

CH3CH2CH3

CH3CH3

Formulas and Properties of “Normal” Alkanes

Homologous Series

Members of the series

differ by one —CH2— group

CH4

CH3CH2CH2CH2CH2CH2CH2CH2CH2CH3

CH3CH2CH2CH2CH2CH2CH2CH2CH3

CH3CH2CH2CH2CH2CH2CH2CH3

CH3CH2CH2CH2CH2CH2CH3

CH3CH2CH2CH2CH2CH3

CH3CH2CH2CH2CH3

CH3CH2CH2CH3

CH3CH2CH3

CH3CH3

Formulas and Properties of “Normal” Alkanesn Molecular Condensed Name Melting Boiling

Formula Structural Formula point (oC) point (oC)

1 CH4 methane -182 -1622 C2H6 ethane -183 -893 C3H8 propane -190 -424 C4H10 butane -138 -15 C5H12 pentane -130 366 C6H14 hexane -95 697 C7H16 heptane -91 988 C8H18 octane -57 1269 C9H20 nonane -51 151

10 C10H22 decane -30 174

Boiling Points of the First Ten Alkanes

All Molecules

Polar Molecules

Molecules containing O-H, N-H, or F-H

Bonds

Dispersion forces

Dipole forces

H-bonding

Hierarchy of Intermolecular Forces

Boiling Points of Straight Chain Alkanes NonPolar Molecules

Effect of Molecular Shapeon Size of Dispersion Force

n-pentane molar mass=72.15

b.p = 36.1 ºC

2-methylbutane molar mass=72.15

b.p = 27.9 ºC

2,2-dimethylpropane molar mass=72.15

b.p = 9.5 ºC

A larger surface-to-surface contact between molecules results in stronger dispersion force attractions and a

higher boiling point.

Drawing Organic Molecules

Lewis Structure

Condensed Structure

Carbon-Skeleton Structure

Molecular Formula C5H12

Alkanes - The Meaning of Line Structures

A. Atoms may freely “rotate” about Single bonds

B. Such rotation is not evident in very simple molecules, but can be demonstrated in models of larger molecules

1. Methane - CH4

Rotation does not change the spacial relationship between hydrogen atoms.

2. Ethane - CH3CH3

Rotation does change the spatial relationship between hydrogen atoms

“Conformations” of Alkanes

Conformations of Alkanes

C. CONFORMATIONS - Different arrangements of atoms in a molecule which can be interconverted by rotation about single bonds

D. In a given sample of a compound, the molecules usually exist as a mixture of conformations. One or two conformations are usually the most stable.

E. Though they may appear different on paper, different conformers represent the same molecule

CH3 CH2 CH2 CH3 CH2 CH2 CH3

CH3

CH3

CH2 CH2

CH3 CH3

CH2 CH2

CH3

CH3 CH2

CH3

CH2

Rotation about a bond is not isomerism.

Isomerism, A Complicating Factor in Organic Chemistry

A. A molecular formula may not convey a unique structure.

B. Isomers - Compounds with identical molecular formulas, but different structural formulas.

C. Example #1: C4H10

Compound

Name Butane Isobutane

Melting point -138 oC -160 oC

Boiling point 0 oC -12 oC

Density 0.601 g/mL 0.557 g/mL

H3CCH2 CH2

CH3

H

CH3C CH3

CH3

Isomerism

Isomerism

n-butane iso-butanebutane 2-methylpropane

Alkane Nomenclature

Alkane Nomenclature

Why is the IUPAC system Necessary ?

Consider the following “carbon hydrides”

CH4 C2H6 C2H4 C2H2

Each formula represents a unique compound.

When structural formulas are considered, however,

C3H7 has only 1 isomer. C4H10 has 2 isomers.

C5H12 has 3 isomers. C6H14 has 5 isomers.

C10H22 has 75 isomers. C30H62 has 4 x 109 isomers.

Basis of the IUPAC System of naming-The Root Name

The root name of a compound is derived from the longest continuous chain of carbon atoms

or the longest continuous chain of carbon atoms containing a given

special structural feature.

1) Common stems are used to indicate the length of carbon chains.

meth-, eth-, pro-, but-, pent-, hex-, hept-, oct-, non-, dec-

2) Common suffixes are use to indicate the class of compounds.

Alkane Alkene Alkyne

C CC CC C

Alkane Nomenclature

“Substituents” may be attached to a carbon chain

1. Substituents which are not pieces of alkanes

2. Substituents which are pieces of alkanes: “Alkyl” groups

Alkane Nomenclature

1. Substituents which are not pieces of alkanes:

-F “Fluoro”- -NO2 “Nitro”-

-Cl “Chloro"- -NH2 “Amino”-

-Br “Bromo”- -OH “Hydroxy”-

-I “Iodo”-

Alkane Nomenclature

2. Substituents which are pieces of alkanes - “Alkyl” groups

Names are derived from the parent alkanes.

C HH

H

H

Parent alkane When attached to another chain

Methane

CH

H

H

Parent Chain

Methyl- BASE NAME

C CH

H

H

H

H

H

Ethane

C CH

H

H

H

H

Parent Chain

Ethyl- BASE NAME

Alkane Nomenclature

2. Substituents which are pieces of alkanes - “Alkyl” groups

Parent alkane When attached to another chain

C CH

H

H

H

H

H

Parent Chain

Propane Propyl- BASE NAME

C

H

H C CH

H

H

H

H

H

C

H

C CH

H

H

H H

C

H

H

Parent Chain Isopropyl- BASE NAME

Alkane Nomenclature

methyl- CH3-

ethyl- CH3CH2-

propyl- CH3CH2CH2-

isopropyl- (CH3)2CH-

n-butyl- CH3CH2CH2CH2-

s-butyl- CH3CH2CHCH3

isobutyl- (CH3)2CH2CH-

t-butyl- (CH3)3CH-

Alkane Nomenclature -Alkyl GroupsName Formula Line structure

C3H7-

C4H9-

C2H5-

CH3-

Alkane Nomenclature -Final Step

Position of Attachment of Substituents is Indicated

1. For carbon chains with double or triple bonds (alkenes and alkynes), the numbering system for the base carbon chain has already been established and does not change.

2. For saturated carbon chains (alkanes) the numbering system for the base carbon chain is determined by the positions of the substituents.

Substituent names (in alphabetical order) and numbers are added (prefixed) to the base name of the compound.

Putting it Together for a Simple Alkane

CH3 CH2 CH2 CH

CH CH3

CH3

CH CH3

CH2

CH3

7 6 5 4 3

2

1

Family

heptane

Longest chain

Base name

Alkyl groups

methylisopropyl

Position of alkyl groups

4- -3-

CH3 CH2 CH2 CH

CH CH3

CH3

CH CH3

CH2

CH3

Alkane Nomenclature Worksheet

Br

1) 2)

3)4)

2-methyl pentane

2,3-dimethyl pentane

2,3,4-trimethyl hexane1-bromo-3,3-dimethyl pentane

H3CCH

CH2CH2

CH3

CH3

H3CCH

CHCH2

CH3

CH3

CH3

3,4-dimethyl hexane

BrCl

5) 2,2,3,3,4,4-hexamethyl pentane

6)

7) 2,2,3-trimethylnonane

8) 3,3-diethyl-4-isopropylheptane

1 1

35

1

5

18

14

1 8

Cycloalkanes

C

C C

H H

H

H

H

H

CC

CC

C

C

H HH

H

H

HH H

HH

H

H

1. Structures are usually abbreviated

2. Planar representations do not represent 3-D shape.

Nomenclature - Cyclic Alkanes

3. Cyclic structures are numbered and named to give the lowest numbers to substituents.

Nomenclature - Cyclic Alkanes

CH3H3C1,2-dimethylcyclopentane 1,2,4-trimethylcyclohexane

CH3

H3C

H3C1 2

1

2 34

1 1

35

9) 10)

1-isopropyl-3-methyl cyclohexane

1,1,3,3-tetramethyl cyclopentane

1

3

1 3

BrCl

Question: Are pentane and cyclopentane isomers ???

C5H12 C5H10