ORGANIC CHEMISTRY (1) ALKANES, ALKENES AND ALKYNES 1

2 Classification of Hydrocarbons Hydrocarbons are made up of only hydrogen and carbon. -ane cyclo__-ane-ene-yl

Hydrocarbons are compounds that contain only carbon and hydrogen atoms.  Alkanes ( C n H 2n+2 ) are hydrocarbons that do not have multiple bonds between carbon atoms Other hydrocarbons may contain double or triple bonds between their carbon atoms.  Alkenes ( C n H 2n ) contain at least one carbon–carbon double bond.  Alkynes ( C n H 2n-2 ) contain at least one carbon–carbon triple bond.  Aromatic compounds contain a special type of ring, the most common example of which is a benzene ring. 3

4  Hydrocarbons are compounds made up of only the elements carbon and hydrogen.  They may be aliphatic or aromatic. Hydrocarbons

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Alkanes are carbon compounds that contain only single bonds. The simplest alkanes are hydrocarbons –only carbon and hydrogen. Hydrocarbons are used mainly as fuels, solvents and lubricants:

A LKANES Only single bonds, C, H Straight chained, branched, cyclic IUPAC Nomenclature “International Union of Pure & Applied Chemistry” Homologous series of Alkanes CH3(CH2)nCH3 -(CH2)- methylene group Isomers (branched alkanes)

1o1o T YPE OF CARBON ATOMS 2o2o 3o3o 4o4o

A LKANE F ORMULAS All C-C single bonds Saturated with hydrogens Ratio: CnH2n+2 Alkane homologs: CH 3 (CH 2 ) n CH 3 Same ratio for branched alkanes =>

12 The suffix “-ane” identifies a molecule as an alkane. Naming Alkanes

C OMMON N AMES Isobutane, “isomer of butane” Isopentane, isohexane, etc., methyl branch on next-to-last carbon in chain. Neopentane, most highly branched Five possible isomers of hexane, 18 isomers of octane and 75 for decane Nomenclature Pentanes =>

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IUPAC N AMES Find the longest continuous carbon chain. Number the carbons, starting closest to the first branch. Name the groups attached to the chain, using the carbon number as the locator. Alphabetize substituents. Use di-, tri-, etc., for multiples of same substituent. =>

L ONGEST C HAIN The number of carbons in the longest chain determines the base name: ethane, hexane. If there are two possible chains with the same number of carbons, use the chain with the most substituents. =>

N UMBER THE C ARBONS Start at the end closest to the first attached group. If two substituents are equidistant, look for the next closest group =>

A LPHABETIZE Alphabetize substituents by name. Ignore di-, tri-, etc. for alphabetizing. 3-ethyl-2,6-dimethylheptane =>

Examples 3-ethyl-7,4- dimethylnonane

P HYSICAL P ROPERTIES OF A LKANES Alkanes are nonpolar, so they dissolve in nonpolar or organic solvents. Alkanes are said to be hydrophobic (“water hating”) because they do not dissolve in water. Boiling Points The boiling points of unbranched alkanes increase with the number of carbon atoms. Branched alkanes have lower boiling points than their unbranched isomers. Melting Points Like their boiling points, the melting points increase with increasing molecular weight. Alkanes with even numbers of carbon atoms pack better into a solid structure, so that higher temperatures are needed to melt them. 20

-The first four alkanes in homological row are gaseous at room temperature. -The unbranched alkanes pentane (C 5 H 12 ) through heptadecane (C 17 H 36 ) are liquids, -Whereas higher homologs are solids. -The boiling points of unbranched alkanes increase with the number of carbon atoms. -Branched alkanes have lower boiling points than their unbranched isomers.

-Solid alkanes are soft, generally low-melting materials. - All alkanes are insoluble in water. Being insoluble, and with densities in the g/mL range, The exclusion of nonpolar molecules, such as alkanes, from water is called the hydrophobic effect. The most alkanes are soluble in organic solvents — diethyl ether, CCl 4, benzol. -Their insolubility increases with the increasing of number of carbon atoms in the molecule.

P REPARATION OF ALKANES AND CYCLOALKANES 1- HYDROGENATION OF ALKENES AND ALKYNES Alkenes and alkynes react with hydrogen in the presence of metal catalysts such as nickel, palladium, and platinum to produce alkanes. General Reaction 23

E XAMPLES : 24

2- REDUCTION OF ALKYL HALIDES Most alkyl halides react with zinc and aqueous acid to produce an alkane. General Reaction 25 Example

3- W URTZ REACTION (A LKYL HALIDES VIA COUPLING ) Two R groups can be coupled by reacting RBr, RCl or RI with Na or K 26 2R-X + 2Na R-R + 2NaX 2CH 3 -CH 2 -CH 2 -Cl + 2Na CH 3 -CH 2 -CH 2 -CH 2 -CH 2 -CH 3 + 2NaCl n-propylchloride n-Hexane

4. A LKYL HALIDES VIA G RIGNARD REAGENT 27 Example

5- ALKYLATION OF TERMINAL ALKYNES 1. Terminal alkyne: an alkyne with a hydrogen attached to a triply bonded carbon. The acetylenic hydrogen is weakly acidic and can be removed with a strong base (e.g. NaNH 2 ) to give an anion (called acetylide ion ). 2. Alkylation: the formation of a new C—C bond by replacing a leaving group on an electrophile with a nucleophile. General Reaction 28

Example 29

6- PREPARATION OF ALKANES AS THE FOLLOWING REACTIONS : 1- Al 4 C 3 + H 2 O CH 4 + Al(OH) 3 2- C + H 2 O CO + H 2 Ni 3H 2 CH 4 3- CO 2 + 4H 2 CH 4 + 2H 2 O 1200 o C 4- C + 2H 2 CH 4 5- CH 3 COONa + NaOH CH 4 + Na 2 CO 3

31 In normal conditions alkanes do not react with acids and alkalis because  -bonds in their molecules are very strong. But alkanes take part in such reactions as: -reactions of the substitution; -reactions of the oxidation; -reactions of the destruction.

Reactions of Alkanes Alkanes found naturally in crude oil (decomposing organic matter w/o oxygen) Conversion into more reactive compounds – Functionalization – E.g. halogenation Homolytic Cleavage of Covalent Bonds Homolytic cleavage creates radicals, requires energy input to break bond H-H  H. + H. DH o = 104 kcal/mol Radical = molecule with an unpaired free electron – Mostly in gas phase or nonpolar solvent Heterolytic cleavage NaCl  Na + + Cl - – Ions are accommodated in polar solvents

Radical Reactions – Halogenation of Alkanes

34 RH + X2 RX + HX uv Δ

Write the propagation steps leading to the formation of dichloromethane CCl 4 ))(CH 2 Cl 2 ), chloroform (CHCl 3 ) and tetrachloromethane 36

37 Alkanes can burn if oxygen is present. As the result H 2 O and CO 2 appear. CH 4 + 2O 2 → CO 2 + 2H 2 O C RACKING IS THE DESTROYING OF ­­ SOME −C−C− AND −C−H BONDS IN THE MOLECULE OF ALKANES AT HIGH TEMPERATURE.

N AMING C YCLOALKANES Cycloalkane usually base compound May be cycloalkyl attachment to chain. =>