Representation Of Molecular FormulaeRepresentation Of Molecular Formulae

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Alkanes are the hydrocarbons of aliphatic row.

General formula of Alkanes CnH2n+2

Alkanes are hydrocarbons in which all the bonds are single covalent bonds (-bonds).

Alkanes are called saturated hydrocarbons.

The simplest one, methane (CH4), is also the most abundant .

Cyclic alkanes have the formula CnH2n

cyclohexane cyclopropane

Here are examples of cyclic alkanes

Cycloalkanes are alkanes in which all or some of the carbon atoms are arranged in a ring

a b c

d e

Practice Exercise: Circle each moleculethat can be classified as an alkane

Alkanes can have either simple (unbranched) or branched carbon chain.

Alkanes with unbranched Carbon chain are called normal or n-alkanes.

In the molecules of alkanes all carbon atoms are in the state of sp3-hybridization.

The distance between two Carbon atoms is 0.154 nm, but the distance between two atoms of Carbon and Hydrogen is 0.110 nm.

The rotation can take place around C—C bonds.

As the result of this rotation the molecule have different conformations (spatial forms).

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

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ATOMIC ORBITALSالمدارات الذرية

S-Orbital

P-OrbitalThe three 2p orbitals are orientated perpendicular to each other

d-Orbital

The length of the band: 1.54 AAngle: 109.5

Hybridization

sp3 hybridization in Methane

sp3 Hybrid Orbitals

• 4 VSEPR pairs – 1 s and 3 p orbitals• Tetrahedral e- pair geometry

• 109.5° bond angle

sp2 hybridization in Ethene

sp2 Hybrid Orbitals

3 VSEPR pairs – 1 s and 2 p orbitalsTrigonal planar e- pair geometry120° bond angle

sp hybridization in Ethyne

2-VSEPR pair -1S and 1P orbital –2sp hybridized orbital Linear electron pair geometry180° bond angle

sp Hybrid Orbitals

Bond lengths of Ethyne, Ethene, and Ethane

This is a straight-chain alkane.

This is a branched-chainalkane.

Alkanes can be represented in several ways.

Primary through Quaternary Carbons

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

Primary through Quaternary Carbons

Isomers are two or more molecules with the same molecular formula but..…

…differ in how the atoms are connected (linkages ) or

…how the atoms are arranged in space (spatial) .

Alkanes and Isomers

Isomers

StereoisomersConstitutional Isomers

(atoms are connected dif f erently)(atoms have the same connectivity but dif f erent spatial arrangements)

Alkanes are characterized by structural and optical isomer.

Structural(Constitutional) isomer is formed by different sequence of carbon atom connections (isomer of the carbon chain). e.g. methane is the only alkane of molecular formula CH4, ethane the only one that is C2H6, and propane the only one that is C3H8. compounds with the same molecular formula but different structures Beginning with C4H10 constitutional isomers are possible; two alkanes have this particular molecular formula. In one, called n-butane, four carbons are joined in a continuous chain. The second isomer has a branched carbon chain and is called isobutane.

Structural isomerism

Chain isomerism

Position isomerism

Functional group isomerism

Different carbon skeletons

Functional groups in different positions

Different functional groups

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Structural Isomers

CH3 O CH3 and CH3 CH2 OH

CH3

CH3

and

Position isomersChain isom

erism

position isomers

CH3 O CH3 and CH3 CH2 OHposition isomers

CH3

CH3

and

2-methylbut-1-ene

2-methylbut-2-ene

3-methylbut-1-ene

2

2

1

3 1

Functional group has a higher priority than branches

Stereoisomerism

Geometrical isomerism

Optical isomerism

Same linking order of atoms

Different spatial (مكاني)arrangements of atoms

Geometrical IsomerismGeometrical Isomerism

e.g. trans-but-2-ene and Cis-but-2-ene

trans cis

Compounds with the same covalent arrangement but different arrangement around a carbon-carbon double bond

Cis-trans isomers must be two different groups on the sp2 carbon.

Both substituent groups are on the same side w.r.t. the axis of the C=C double bond

Both substituent groups are on the opposite sides w.r.t. the axis of the C=C double bond

cannot be inter-converted at lower temperatures

C CH3C

H H

HNo cis-trans isomers possible

Rotation about the axis of a double bond through an angle of 90o results in the breaking of the π bond

maximum overlap of pz orbitals

minimum overlap of pz orbitals

Are these isomers?

If they are isomers, then whattype of isomers are they?

2pent-2-ene

)2E-(pent-2-ene

)2Z-(pent-2-ene

Geometrical IsomerismGeometrical Isomerism

trans

cis

E/Z notation

If there are three or four different groups attached to the C atoms of C=C double bondE/Z notation rather than the cis/trans notation is used to name the stereoisomers of a molecule.E : in opposition to transZ : together (same side) cis

 It is an extension of cis /trans  notation (which only describes relative stereochemistry) that can be used to describe double bonds having three or four substituents  

In simple cases, such as 2-butene, Z corresponds to cis and E to trans. However, that is not a ruleThe real advantage of the E-Z system is that it will always work. In contrast, the cis-trans system breaks down with many ambiguous cases. Example...1-bromo-2-chloro-2-fluoro-1-iodoethene.

Drow five constitutional isomers with the same chemical formula as heptane C7H16. Use condensed structure formula