HydrocarbonsAliphatic Straight chainSaturatedAlkanesUnsaturatedAlkenesAlkynesCyclic Aromatic Cyclic

Bonding in Organic Compounds In organic compounds, carbon has 4 valence electrons and hydrogen has 1 carbon achieves an octet by forming four bonds, with each line representing 2 electrons

Representation Of Molecular Formulae 5

Expanded, Condensed, and Skeletal Structures A hexane molecule can be represented in several ways: a molecular formula a ball-and-stick model an expanded structural formula a condensed structural formula a skeletal formula

ATOMIC ORBITALS المدارات الذرية S-Orbital

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

d-Orbital

We can explain the tetrahedral geometry of methane by imagining that the 2s orbital and the three 2p orbitals of carbon hybridize to form four identical sp 3 hybrid orbitals We can divide the hybridization process into two steps. Hybridization 1. First, an electron in the 2s orbital is “promoted” to a vacant 2p orbital to produce an excited state of carbon. The 2s orbital in this state has only one electron. 2. Second, the half-filled 2s orbital and the three half-filled 2p orbitals mix to form new sp 3 hybrid orbitals (pronounced “s-p-three,” not “sp cubed”).

(a) The original set of four atomic orbitals on carbon are mixed, or hybridized, to give four new sp³-hybridized atomic orbitals. (b ) We have represented the new hybrid orbitals with a new color to emphasize the notion that the hybrid orbitals replace the original unhybridized orbitals. sp 3 - Hybridized Carbon Atom

sp 3 -Hybridized Carbon in Methane Bonding and Structure of Ethane and Conformations of Ethane (a) The bonding molecular orbital of the C—C bond in ethane is cylindrically symmetrical. (b) Rotation of the two methyl groups about the C—C bond axis maintains the bond, but changes the relative positions of the C—H bonds.

sp 2 hybridization of carbon in ethene Each carbon atom in ethene is bonded to three other atoms, we need three s bonds. We hybridize carbon by “mixing” a 2s orbital and two 2p orbitals to obtain three sp 2 hybrid orbitals (pronounced “s-p- two”). The third 2p orbital remains unchanged. The three sp 2 hybrid orbitals have the same shapes and energies. The orbitals differ only in their position in space. They lie in a plane and are directed to the corners of an equilateral triangle—therefore separated by 120°—to achieve maximum separation of the electrons In contrast to the rotation that occurs around the C—C bond of ethane, no rotation occurs around the C=C bond of ethylene. Rotation around the C=C internuclear axis would not disrupt the sp 2 –sp 2 σ bond. However, this motion would destroy the overlap of the two 2p orbitals and break the π bond.

sp hybridization of carbon in ethyne We next consider the bonding in ethyne, a molecule in which each carbon atom is bonded to two atoms by s bonds. Let’s suppose that is composed of two methine (CH) units. Since each carbon atom of acetylene is connected to two other atoms, a carbon atom and a hydrogen atom, each methine unit requires two π bonds.

Hybridization sp 3 Tetrahedral sp2 Trigonal Planar sp Linear

 Alkanes are the hydrocarbons of aliphatic row.  General formula of Alkanes CnH2n+2  alkane molecules only contain C—C and C—H Bonds  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 cyclic alkanes have two less hydrogen atoms than the open chain are named by using the prefix cyclo before the name of the alkane chain with the same number of carbon atoms propane, C 3 H 8 cyclopropane, C 3 H 6

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).

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 compounds with the same molecular formula but different structural formula …differ in how the atoms are connected (linkages ) or …how the atoms are arranged in space (spatial). Alkanes and Isomers ex write the structural formulas for three isomers of pentane

 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 (Butane) 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.

TYPES OF ISOMERISM Occurs due to the restricted rotation of C=C double bonds... two forms - CIS and TRANS STRUCTURAL ISOMERISM STEREOISOMERISM GEOMETRICAL ISOMERISM OPTICAL ISOMERISM CHAIN ISOMERISM Same molecular formula but different structural formulae Occurs when molecules have a chiral centre. Get two non- superimposable mirror images. Same molecular formula but atoms occupy different positions in space. POSITION ISOMERISM FUNCTIONAL GROUP ISOMERISM

Structural isomerism Chain isomerism Position isomerism Functional group isomerism Different carbon skeletons Functional groups in different positions Different functional groups

31 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?

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

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

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