ORGANIC CHEMISTRY

By Mubarak O. AMEEN 1

•Aromatic Hydrocarbons

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• Aromatic hydrocarbons are benzene and benzenoids compounds (i.e. compounds resembling benzene)

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• Benzene C6H6 & benzenoids are unusually stable because of large electron energies are called aromatic compounds.

• Compounds like benzene, which have relatively few hydrogens in relation to the number of carbons, are typically found in oils produced by trees and other plants.

• Early chemists called such compounds aromatic compounds because of their pleasing fragrances.

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• To be classified as aromatic, a compound must meet the following criteria: – The compound must be cyclic.

– The molecule must be planar.

– It must have an uninterrupted cyclic cloud of pi electrons

– The cloud must contain an odd number of pairs of electrons. Hückel’s rule i.e. 4n + 2

• The rule states that “for a planar, cyclic compound to be aromatic, its uninterrupted cloud must contain (4n + 2) pi electrons, where n is any whole number.

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Nomenclature of substituted Benzenes • Some monosubstituted benzenes are named

simply by stating the name of the substituent, followed by the word “benzene.”

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• Some monosubstituted benzenes have names that incorporate the name of the substituent.

• Unfortunately, such names have to be memorized.

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• With the exception of toluene, benzene rings with an alkyl substituent are named as alkyl-substituted benzenes or as phenyl-substituted alkanes.

Reactions of Benzene • Benzene mainly undergoes substitution reactions • The following are the five most common electrophilic aromatic

substitution reactions: • Halogenation: A bromine (Br), a chlorine (Cl), or an iodine (I)

substitutes for a hydrogen. • Nitration: A nitro (NO2) group substitutes for a hydrogen. • Sulfonation: A sulfonic acid (SO3H) group substitutes for a

hydrogen. • Friedel–Crafts acylation: An acyl (RC=O) group substitutes for a

hydrogen. • Friedel–Crafts alkylation: an alkyl (R) group substitutes for a

hydrogen.

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Study Questions • Draw the structure of each of the following compounds:

– 2-phenylhexane

– 3-benzylpentane

– benzyl alcohol

– bromomethylbenzene

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•Alkanol (Alcohols)

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Alcohols • Alcohols are compounds in which a hydrogen of

an alkane has been replaced by an OH group.

• General formula CnH2n+1OH

• Alcohols are classified as primary, secondary, or tertiary, depending on whether the OH group is bonded to a primary, secondary, or tertiary carbon—the same way alkyl halides are classified.

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• The common name of an alcohol consists of the name of the alkyl group to which the OH group is attached, followed by the word “alcohol.”

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Systematic Nomenclature

• The functional group is the center of reactivity in a molecule.

• In an alcohol, the OH is the functional group.

• The IUPAC system uses a suffix to denote certain functional groups.

• The systematic name of an alcohol, for example, is obtained by replacing the “e” at the end of the name of the parent hydrocarbon with the suffix “ol.”

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• When necessary, the position of the functional group is indicated by a number immediately preceding the suffix.

• However, names with the number preceding the name of the alcohol have been in use for a long time, so those are the ones most likely to appear in the – Literature – on reagent bottles; and – on standardized tests.

• They are also the ones that appear most often in most books.

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• The following rules are used to name a compound that has a functional group suffix:

The parent hydrocarbon is the longest continuous chain containing the functional group.

The parent hydrocarbon is numbered in the direction that gives the functional group suffix the lowest possible number.

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If there is a functional group suffix and a substituent, the functional group suffix gets the lowest possible number.

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If the same number for the functional group suffix is obtained in both directions, the chain is numbered in the direction that gives a substituent the lowest possible number.

Notice that a number is not needed to designate the position of a functional group suffix in a cyclic compound, because it is assumed to be at the 1-position.

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If there is more than one substituent, the substituents are cited in alphabetical order.

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Types of alcohol

• Monohydric alcohol: alcohol with one OH group

• Polyhydric alcohol: alcohol with more than one OH group

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•Do you have any question?

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Study questions • Draw the structures of a homologous series of alcohols that

have from one to six carbons, and then give each of them a common name and a systematic name.

• Give each of the following compounds a systematic name, and indicate whether each is a primary, secondary, or tertiary alcohol:

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Study questions

• Write the structures of all the tertiary alcohols with molecular formula C6H14O, and give each a systematic name.

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Ethanol

• The most useful alcohol C2H5OH

• It can be prepared by

– Hydrolyzing iodoethane by alkali

– Hydrolyzing ethanal with nascent hydrogen

– Reducing ethanal with nascent hydrogen

• Industrially it can be prepared

– From Ethane

– By fermentation

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Properties

• It reacts with sodium and potassium to form ethoxides

– 2C2H5OH + 2Na 2C2H5ONa + H2

• It reacts with phosphorus V chloride to form hydrogen chloride and chloroethane

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Uses

• As solvent

• Used to produce halides, etc

• Used as fuel

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•Any questions?

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•Carboxylic acids (Alkanoic acid)

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