By

Dr. Nahed Nasser

CONTENTSCONTENTS

>> Hydrocarbons

>> Structure of alkanes

>>Representation of M F

>> Classes of carbon and hydrogen atoms

>>Hybridisation of orbitals

>>Structural isomerism in alkanes

>>Alkyl Groups and Structural isomerism

>>IUPAC nomenclature of alkanes

>>Physical properties of alkanes

>>Preparation of alkanes

>>Reactions of alkanes

>>Pollution

>>Cycloalkanes, nomeclature , cis / trans isomerism and reactions

THE CHEMISTRY OF ALKANESTHE CHEMISTRY OF ALKANES

Alkanes : CnH2n+2

NameMolecular Formula

MethaneCH4

EthaneC2H6

PropaneC3H8

ButaneC4H10

PentaneC5H12

HexaneC6H14

HeptaneC7H16

OctaneC8H18

NonaneC9H20

DecaneC10H22

3

REPRESENTATION OF MOLECULAR FORMULAS

4

5

Drawing alkanesDrawing alkanes

n-Pentane

6

Classes of carbon and HydrogenClasses of carbon and Hydrogen

• Primary carbon : CH3-CH2-CH3

• Secondary carbon : CH3-CH2-CH3

• Tertiary carbon : (CH3)2-CH-CH3

• Hydrogens are also referred to as 1º, 2º or 3º according to the type of carbon they are bonded to.

7

SpSp33 HYBRIDISATION OF ORBITALS HYBRIDISATION OF ORBITALS

The electronic configuration of a carbon atom is 1s22s22p2

1 1s

22s

2p

If you provide a bit of energy you can promote (lift) one of the s electrons into a p orbital. The

configuration is now 1s22s12p3

1 1s

22s

2p

The process is favourable because the arrangement of electrons; four unpaired and with less repulsion is more

stable8

The four orbitals (an s and three p’s) combine or HYBRIDISE to give four new orbitals. Because one s and three p orbitals are used, it is called sp3

hybridisationAll four orbitals are equivalent in energy, each with 25 % S character and

75 % P character.

2s22p2 2s12p3 4 x sp3

9

In ALKANES, the four sp3 orbitals of carbon repel each other into a TETRAHEDRAL

arrangement with bond angles of 109.5º.

Each sp3 orbital in carbon overlaps with

the 1s orbital of a hydrogen atom to form

a C-H bond.

THE STRUCTURE OF ALKANESTHE STRUCTURE OF ALKANES

109.5º

10

The length of the bond: 1.54 AAngle: 109.5

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Structural Isomerism in alkanesStructural Isomerism in alkanes

Different compounds with identical molecular formulas are called ISOMERS and the phenomenon is called ISOMERISM

Butane IsobutaneBoiling point 0 -121Melting point -138 -14512

Pentane, C5H12 has three chain isomers

Exercise :Draw all possible structural isomers for the M.F. C6 H14 . And name them Or

How many isomeric structures can be exhibited by the M.F. C6 H14 ?

CH3

CH

CH3H2CH3C

CH3

C CH3H3C

CH3

H3C

H2C

CH2

H2C

CH3

n-Pentane2-Methyl-butane

Isopentane2,2-Dimethyl-propane

Neopentane

13

Alkyl groupsAlkyl groups

• Alkyl groups are formed by loss of a hydrogen atom from the corresponding alkane

• ( e.g. CH4 Methane – 1 H = CH3 Methyl group )

• Alkyl groups are named by dropping the -ane suffix of the alkanes and adding the suffix -yl. Methane becomes a methyl group, ethane an ethyl group, etc.

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Structural isomerism in alkyl groupsStructural isomerism in alkyl groups

• Propyl group C3H7 (can give two isomeric

alky groups)

OR

CH3-CH2-CH2- CH3-CH

CH3

n-Propyl Isopropyl

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Butyl group C4H9

It can exist in three isomeric forms

CH3-CH2-CH2-CH2- CH3-CHH2C

CH3

H3C C

CH3

CH3

n-Butyl Isobutyl tert.Butyl

16

IUPAC NOMENCLATURE OF BRANCHED-IUPAC NOMENCLATURE OF BRANCHED-CHAIN ALKANESCHAIN ALKANES

1- Locate the longest continuous chain of carbon atoms; this chain determines the parent name for the alkane.

Sometimes, you may need to go around corners and zigzag to find the longest (parent) chain. (the parent chain is in blue):

• If the parent chain for example has 6 carbon atoms, therefore, it is

a derivative of hexane and if it has 4 carbon atoms it is derivative of butane and so on .

H3CHC CH CH2

H2C CH2 CH3

CH3

CH3CH3CH2CH2CH2CHCH3

CH3

CH3CH2CH2CH2CHCH3

CH2

CH3

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2 -Number the longest chain beginning with the end of the chain nearer to the substituent.

CH3CH2CH2CH2CHCH3

CH3

CH3CH2CH2CH2CHCH3

CH2

CH3

Substituent

Substituent

1

2

345 67 5

1

2346

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3 -Use the numbers obtained by application of rule 2 to designate the location of the substituent group.

The parent name is placed last; the substituent group, preceded by the number indicating its location on the chain, is placed first.19

4 .When two or more substituents are present, give each substituent a number corresponding to its location on the longest chain.

The substituent groups are listed alphabetically regardless of their order of occurrence in the molecule. Cl is called chloro, Br called bromo, I called iodo, NO2 called nitro, CN called cyano

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5 (When two or more substituents are identical, indicate this by the use of the prefixes di-, tri-, tetra-, and so on.

In case of deciding alphabetical order of many substituent disregard multiplying prefixes such as “di”and “tri”, “tetra”, “penta” ,

.…21

6 (When two substituents are present on the same carbon, use the number twice.

22

CH3

CH3CCH2 CH2CH2CH3

CH2

CH3

3-Ethyl-3-methylhexane

7 .When two chains of equal length compete for selection as the parent chain, choose the chain with the greater number of substituents.

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8 .When branching occurs at an equal distance from both ends of the longest chain, choose the name that gives the lower number at the first point of difference.

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Summary of IUPAC system of Summary of IUPAC system of nomenclaturenomenclature

1. Find and name the longest continuous carbon chain.2. Identify and name groups attached to this chain.3. Number the chain consecutively, starting at the end

nearest a substituent group. 4. Designate the location of each substituent group by an

appropriate number and name.5. Assemble the name, listing groups in alphabetical order.6. The prefixes di, tri, tetra etc., used to designate several

groups of the same kind, are not considered when alphabetizing.

7. Halogen substituents are easily accomodated, using the names: fluoro (F-), chloro (Cl-), bromo (Br-) and iodo (I-).

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Examples of the IUPAC Rules in Practice

• By inspection, the longest chain is seen to consist of six carbons, so

the root name of this compound will be hexane. A single methyl

substituent (colored red) is present, so this compound is a

methylhexane. The location of the methyl group must be specified,

since there are two possible isomers of this kind. The IUPAC name

is thus 3-methylhexane.26

Thus the parent chain will be the one with 4 substituents and the correct IUPAc name

of this compound is:

33--Ethyl-2,2,5-trimethylhexaneEthyl-2,2,5-trimethylhexane

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Important notesImportant notes• The common names isopropyl, isobutyl, sec-butyl, tert-butyl are approved by the

IUPAC for the substituted groups. • Substituent groups are cited in the name in alphabetical order, regardless of their order of

occurrence in the molecule. Prefixes di, tri, tetra, and (di, tri, tetra, and (terttert written in italics and separated from the name by a hyphen) are ignoredare ignored, but prefixes iso, neo, and cyclo are not!

Thus “tert-butyl” precedes “ethyl”, but ethyl preceeds “isopropyl”

• 3-ethyl comes before 2,2-dimethyl • 4-hexyl comes before 2,3-diisopropyl • 3-Tert-butyl comes before 3-isopropyl

4-Isopropyl-3-methyl-decane

1

234

5

6

7

8

9

10

1234

56

7

8

9

10

6-tert-Butyl-2-methyl-decane

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Physical PropertiesPhysical Properties

• Methane, ethane, propane, and butane are gases; pentane through hexadecane are liquids; the homologues larger than hexadecane are solids. • The boiling points and melting points of alkanes increase with molecular weight. • Branching reduces the boiling point, the more branching the lower the boiling point. •Alkanes are non- polar so are immiscible with water , they are soluble in most organic solvents.

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Preparation of alkanesPreparation of alkanes

1- Hydrogenation of unsaturated hydrocarbon:1- Hydrogenation of unsaturated hydrocarbon:

2- Hydrolysis of Grignard reagent2- Hydrolysis of Grignard reagent

CH2 CH2

Ni or Pd or Pt / H2

200, 300CH3 CH3

CH3CH2Br + Mg2+ Dry ether CH3CH2MgBr

Grignard reagent

CH3CH2MgBrH3O

+

CH3CH3 + Mg(OH)Br

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3- Reduction of alkyl halides3- Reduction of alkyl halides

a) by metal and acid or by metal hydrides

b) By sodium metal (Coupling reaction)

c) By lithium dialkyl cuprate

CH3 Br2 + 2 Na CH3 CH3 + 2 NaBr

(CH3CH2)2CuLi + CH3Br CH3CH2CH3

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CH3CH2CH2Br + Zn H CH3CH2CH3 + ZnBr2

CH3CH2CH2CH2Br1) LiAlH4 / ether

2) H3O

CH3CH2CH2CH3

Reactions of alkanes Reactions of alkanes • Chemically alkanes are very unreactive and stable at room

temperature towards acids , bases and most reactive metals.

• Despite their relative inertness ( thus they known as paraffines i.e lacking affinity) , alkanes undergo several important reactions that are discussed in the following section.

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• 1- Halogenation1- Halogenation• Halogenation is the replacement of one or more hydrogen atoms in an organic

compound by a halogen (fluorine, chlorine, bromine or iodine). • The halogenation of an alkane appears to be a simple substitution reaction in which a C-H bond

is broken and a new C-X bond is formed; the reaction takes place in presence of heat or UV light ( no reaction in the dark)

• If there is one type of the carbon atoms in the molecule (e.g. methane and ethane)

• If there are different types of carbon atoms in the molecule (Selectivity issue)

When alkanes larger than ethane are halogenated, isomeric products are formed. Thus chlorination of propane gives both 1-chloropropane and 2-chloropropane as mono-chlorinated products.

RH + X2

Heat

or UV lightRX + HX

Alkyl halide

X2 = Cl2 or Br2

H

C HH

H

+ Cl ClHeat or UV light

CH3Cl + CH2Cl2 + CHCl3 + CCl4 + HClexcess

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2- combustion of alkanes 2- combustion of alkanes (burning them-destroying the whole molecule )

160 kcal\mol for each methylene group (CH2)

CH4 + 2 O2 CO2 + 2 H2O + 213 Kcal\mol

C2H6 + 7/2 O2 2 CO2 + 3 H2O + 373 Kcal\mol

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+ Br BrHeat or UV lightH3C CH3

H3C CH3

Br

+

H3C

Br

Major MinorPropane

1 2°

° 1°

Processes involving combustion give rise to a variety of pollutants...

power stationsSO2 emissions produce acid rain

internal combustion enginesCO, NOx and unburnt hydrocarbons

RemovalSO2react emitted gases with a suitable compound (e.g. CaO)

CO and NOxpass exhaust gases through a catalytic converter

Catalytic convertersIn the catalytic converter ...CO is converted to CO2

NOx are converted to N2

Unburned hydrocarbons are converted to CO2 and H2O

e.g.2NO + 2CO ———> N2 + 2CO2

•Catalysts are made of finely divided rare metals Rh, Pd, Pt •Leaded petrol must not pass through the catalyst as the lead deposits on the

catalyst’s surface and “poisons” it, thus blocking sites for reactions to take place .

POLLUTIONPOLLUTION

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CycloalkanesCycloalkanes      •Cycloalkanes are alkanes that have carbon atoms

forming rings (called alicyclic compounds)•Simple cycloalkanes have the formula (CH2)n, or CnH2n •Nomenclature of Unsubstituetd Cycloalkanes COMPOUNDS

1 .Cycloalkanes with only one ring:

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Naming Substituted CycloalkanesNaming Substituted Cycloalkanes

•Count the number of carbon atoms in the ring and the number in the largest substituent chain. If the number of carbon atoms in the ring is equal to or greater than the number in the substituent, the compound is named as an alkyl-substituted cycloalkane. •For an alkyl- or halo-substituted cycloalkane, start at a point of attachment as C1 and number the substituents on the ring so that the second substituent has as low a number as possible.•Number the substituents and write the name with the substituents in alphabetical order

37

38

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• However if the alkyl sustituent is large and/or complex, the ring may be named as a substituent group on an alkane.

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CH2CH2CH2CH2CH3

1-cyclobutylpentane

2

3

1,3-Dicyclohexylpropane

1

41

Cis-Trans Isomerism in CycloalkanesCis-Trans Isomerism in Cycloalkanes

• Rotation about C-C bonds in cycloalkanes is limited by the ring structure

• There are two different 1,2-dimethylcyclopropane isomers, one with the two methyls on the same side (cis) of the ring and one with the methyls on opposite sides (trans)

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Reactions of cycloalkanesReactions of cycloalkanes• Less stable rings

• More stable 5 and 6 rings

HICH3CH2CH2I

H2SO4 / H2OCH3CH2CH2OH

H2/Ni

Heat or UVCH3CH2CH2CH3

Br2/CCl4AlBr3

BrCH2CH2CH2CH2Br

CH3

Br2/UV or Heat

CH3

Br

Cl2/heat or UVCl

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