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Leccture 2

1) Continue introduction:

Hybridization

2) Hydrocarbons (alkanes):

- Nomenclature

- Classification of carbons

- Physical properties

- Reactions

- Sources and importance

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Hybridization

Shape of orbitals: S-orbital (sphere) P-orbital (dumbell-shape)

Lope: high probability of finding e-

Node: zero probability of finding e- Lope

Node

Sigma bond σ Pi bond π

-linear overlapping (strong) -lateral overlapping (weak)

+

+

Hybridization of Atomic Orbitals:

Three perpendicular P orbitals; Px, Py, Pz in C, N or O before the

reaction

Sp3 Hybrid Orbitals

Combination of one S orbital and three P orbitals forms four

equivalent sp3 hybrid orbitals make 4 Sigma bonds with 4 H

atoms forming tetrahedral CH4 (angle 109.2) H

HH

H

Ratio between S character and P character is 1:3

sp2 Hybrid Orbitals

CH2=CH2 molecule, each C has three Sp2 orbital two form 2 sigma

bond with 2 H and a sigma bond with Sp2 orbital of the 2nd C. The

remaining unhybridized P orbitals make lateral overlapping to

form a pi- bond. The molecule is planar (angle 120) covered by 2

electron clouds (above and below).

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C

H

H

C

H

H

e cloud above

e cloud below Ratio between S character and P character is 1:2, so percent of S

character increases (bonds become shorter)

Sp Hybrid Orbitals

C2H2 molecule, each C has two Sp orbitals forming 2 sigma bond

with H, and a sigma bond with Sp orbital of the 2nd C. The

remaining unhybridized two P orbitals (Px, Pz), make lateral

overlapping to form two pi-bonds, surrounding the molecule by a

cylinder of electron cloud. Molecule is linear (angle 180).

C C HH

PxPz

Ratio between S character and P character is 1:1, so percent of S

character increases (bonds are the shortest)

Question:

Page 59, 1.63

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Hydrocarbons

A hydrocarbon is an organic compound composed of carbons and

hydrogens only.

Hydrocarbons are classified to saturated and unsaturated.

Alkanes are saturated hydrocarbons where all the bonds are C-C

single bonds and carbons are sp3 hybridized, while alkenes,

alkynes and aromatic hydrocarbons are unsaturated

hydrocarbons because they contain double or triple bonds and

carbons are sp2 or sp hybridized.

Alkanes

We often refer to alkanes as aliphatic hydrocarbons because there

higher members resemble fats and oil (Greek: aleiphar, fat or oil).

Alkanes have the general formula CnH2n+2. Thus, given

number of carbons in an alkane, we can determine the number of

hydrogens in the molecule. For example, octane, with eight

carbons must have 2x8+2=18 hydrogens and its molecular formula

C8H18.

1] Nomenclature

IUPAC nomenclature of unbranched alkanes

Chemists have adopted a set of rules established by the

International Union of Pure and Applied Chemistry (IUPAC). The

IUPAC name of unbranched chain of alkanes consists of 2

parts:

1) Prefix: indicates the number of carbons Table 2

2) Infix and suffix: ane to show that the compound is saturated

hydrocarbon.

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Table 2

Prefix Number

of carbons

Prefix Number

of carbons

Metha- 1 Octa- 8

Etha- 2 Nona- 9

Propa- 3 Deca- 10

Buta- 4 Undeca- 11

Penta- 5 Dodeca- 12

Hexa- 6 Trideca- 13

Hepta- 7 Tetradeca- 14

Examples:

Methane: CH4, ethane: CH3CH3, propane: CH3CH2CH3,

Hexane: CH3 (CH2)4CH3

IUPAC nomenclature of alkyl substituents (branches):

Alkyl group (R-) is an alkane with removal of one hydrogen. We

name alkyl substituent by replacing –ane by -yl :

Examples:

Methyl: CH3-, ethyl: CH3CH2-, propyl: CH3CH2CH2-,

Hexyl: CH3 (CH2)4CH2-

IUPAC nomenclature of branched alkanes:

1) Select the longest chain of carbon atoms as the parent chain, its

name becomes the main name. If there are 2 chains equal in

length; select the chain with more substituents (branches).

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2) Make numbering for this chain (main chain) from the side

closer to 1st substituent (1st branch), if 1st substituent has the

same number from both sides; make numbering from the side

closer to 2nd substituent and so on…(e.g.3). If two substituents

have the same numbering from both sides; make numbering from

the side that gives substituent of lower alphabetical order least

number (di, tri, tetra not included in alphabetical order).

3) Locate and nominate substituents, and write them in

alphabetical order before the main name (di, tri, tetra not included

in alphabetical order).

Examples:

Example 1:

Name: 2-methylpropane, N.B.: dash is written between

number and letter.

Example 2:

Name: 2-methylpentane

CH3CHCH3

CH3

1 2 3

CH3CH2CH2CHCH3

CH3

5 4 3 2 1

not 1 2 3 4 5

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Example 3:

Name: 2,3,5-trimethylhexane, N.B.: comma is written between

two numbers.

Example 4:

Name: 3-ethyl-5-methylheptane

Example 5:

Name: 4-ethyl-2,2-dimethylhexane.

CH3CHCHCH2CHCH3

CH3CH3

CH3

CH3CH(CH3)CH(CH3)CH2CH(CH3)2

CH3C(CH3)2CH2CH(CH2CH3)2

CH3CHCH2CHCH2CH3

CH3

CH3

CH2CH3

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Naming of some complex alkyl groups: Table2.3 page 69

CH3- methyl

CH3CH2- ethyl

CH3CH2CH2- propyl

1-methylethyl(isopropyl)

CH3CH2 CH2CH2- butyl

CH3CH2CH

3 2 1

CH3

1-methylpropyl(sec-butyl)

CH3CHCH2

3 2 1

CH3

2-methylpropyl(isobutyl)

CH3C

CH3

CH3

1,1-dimethylethyl(tert-butyl)

Problems:

1 2 3

CH3CH

CH3

2 1

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2] Classification of carbons: P.72 in textbook:

Primary carbon (1ry): carbon attached to one carbon, or not

attached to any carbon.

Secondary (2ry) carbon: carbon attached to two carbons.

Tertiary (3ry) carbon: carbon attached to three carbons.

Quaternary (4ry) carbon: carbon attached to four carbons.

3] Nomenclature of cycloalkanes:

Cycloalkanes are alkanes present in a cyclic form.

To nominate a cycloalkane:

1) Count number of carbons in the cycle and name it like in alkane

and add prefix “cyclo”, e.g. cycolpentane, cyclohexane……

2) Begin numbering from one substituent and go in the direction

to give second substituent least numbering. If two substituents

have the same number priority is given to alphabetical order.

Examples:

1) 1-methylethylcyclopentane

2) 1-Ethyl-4-methylcyclohexane

Exercise: P.107, item: 2.19, 2.20 (except g,h), 2.21

1 2

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3] Physical properties of alkanes:

Alkanes containing 1 to 4 carbons are gases.

Alkanes containing 5 to 17 carbons are liquids.

Alkanes containing more than 17 carbons are waxy solids.

4] Reactions:

Oxidation (combustion):

CH4 + 2O2 CO2 + 2H2O + heat

CH3CH2CH3 + 5O2 3CO2 + 4H2O + heat

5] Sources and importance of alkanes: P.101 in textbook.

a) Natural gas: contains 90% methane.

b) Petroleum: by distillation gives many compounds used in

industry, the most applicable is gasoline and kerosene.