HydrocarbonsAliphatic Straight chainSaturatedAlkanesUnsaturatedAlkenesAlkynesCyclic Aromatic Cyclic

Representation Of Molecular Formulae 4

 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 (CH 4 ), is also the most abundant.

Cyclic alkanes have the formula C n H 2n Cycloalkanes are alkanes in which all or some of the carbon atoms are arranged in a ring

Practice Exercise: Circle each molecule that 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 nm, but the distance between two atoms of Carbon and Hydrogen is 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 sp 3 orbitals of carbon repel each other into a TETRAHEDRAL arrangement with bond angles of 109.5º. Each sp 3 orbital in carbon overlaps with the 1s orbital of a hydrogen atom to form a C-H bond. The Structure Of Alkanes 109.5º 9

ATOMIC ORBITALS المدارات الذرية

S-Orbital

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

d-Orbital

The length of the band: 1.54 A Angle: 109.5

Hybridization sp 3 Tetrahedral sp2 Trigonal Planar sp Linear

sp 3 hybridization in Methane

sp 3 Hybrid Orbitals 4 VSEPR pairs – 1 s and 3 p orbitals Tetrahedral e - pair geometry 109.5° bond angle

sp 2 hybridization in Ethene

sp 2 Hybrid Orbitals 3 VSEPR pairs – 1 s and 2 p orbitals Trigonal planar e - pair geometry 120° bond angle

sp hybridization in Ethyne

2-VSEPR pair -1S and 1P orbital –2sp hybridized orbital Linear electron pair geometry 180° bond angle sp Hybrid Orbitals

Bond lengths of Ethyne, Ethene, and Ethane

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

 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 CH 4, ethane the only one that is C 2 H 6, and propane the only one that is C 3 H 8. compounds with the same molecular formula but different structures  Beginning with C 4 H 10 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

30 Structural Isomers

Position isomers Chain isomerism position isomers

2-methylbut-1-ene 2-methylbut-2-ene 3-methylbut-1-ene 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 Isomerism e.g.trans-but-2-ene and Cis-but-2-ene transcis 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 sp 2 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 No cis-trans isomers possible

Rotation about the axis of a double bond through an angle of 90 o results in the breaking of the π bond maximum overlap of p z orbitals minimum overlap of p z orbitals

Are these isomers?

If they are isomers, then what type of isomers are they?

2 pent-2-ene (2E)-pent-2-ene (2Z)-pent-2-ene Geometrical Isomerism trans cis

E/Z notation If there are three or four different groups attached to the C atoms of C=C double bond E/Z notation rather than the cis/trans notation is used to name the stereoisomers of a molecule. E : in opposition to  trans Z : 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 rule The 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 C 7 H 16. Use condensed structure formula