Organic Chemistry (2st semester) Lecture Two 2016 Title Lecture Alkane and Methane

Hydrocarbons Organic compounds contain only two elements, hydrogen and carbon, and hence are known as hydrocarbons. On the basis of structure, hydrocarbons are divided into two main classes, aliphatic and aromatic. Aliphatic hydrocarbons are further divided into families: 1-Alkanes 2-Alkenes 3-Alkynes, 4-Cyclic analogs

Structure of methane The simplest member of the alkane family and, indeed, one of the simplest of all organic compounds is methane, CH 4.

Physical properties Methane is a gas at ordinary temperatures. Because the methane molecule is highly symmetrical as a result, the molecule itself is non polar. Attraction between such non-polar molecules is limited to van der Waals forces. Melting and boiling occur at very low temperatures: m.p -183, b.p Methane is colorless and, when liquefied, is less dense than water (sp.gr. 0.4).slightly soluble in water, but very soluble in organic liquids such as gasoline, ether, and alcohol

Reactions (1)Oxidation. Heat of combustion Burning of hydrocarbons takes place only at high temperatures, as provided, for example, by a flame or a spark. Once started, however, the reaction gives off heat which is often sufficient to maintain the high temperature and to permit burning to continue The heat of combustion: the quantity of heat evolved when one mole of a hydrocarbon is burned to carbon dioxide and water (; for methane its value is 213 kcal.)

2- Chlorination: a substitution reaction Under the influence of ultraviolet light or at a temperature of a mixture of the two gases, methane and chlorine, reacts vigorously to yield hydrogen chloride and a compound of formula CH 3 Cl. We say that methane has undergone chlorination, and we call the product, CH 3 C1, chloromethane or methyl chloride (CH 3 = methyl).

Chlorination is a typical example of a broad class of organic reactions known as substitution Reaction with other halogens: halogenation: as follow Reactivity of halogens F 2 > Cl 2 > Br 2 > I 2

Reaction mechanisms The detailed, step-by-step description of a chemical reaction. By mechanisms knowing how a reaction takes place, we can make changes in the experimental conditions not by trial and error, but logically that will improve the yield of the product we want, or that will even alter the course of the reaction completely and give us an entirely different product.

Transition state Clearly, the concept of act is to be our key to the understanding of chemical reactivity. To make it useful, we need a further concept: transition state, A chemical reaction is presumably a continuous process involving a gradual transition from reactants to products This intermediate structure is called the transition state; its energy content corresponds to the top of the energy

Just as ∆H is the difference in energy content between reactants and products, so E act. is the difference in energy content between reactants and transition state.

The transition state concept is useful for this reason: we can analyze the structure of the transition state very much as though it were a molecule, and attempt to estimate its stability. Any factor that stabilizes the transition state relative to the reactants tends to lower the energy of activation; that is to say, any factor that lowers the top of the energy hill more than it lowers the reactant valley reduces the net height we must climb during reaction.

let us take as an example the transition state for the abstraction of hydrogen from methane by a halogen atom, and see where a little thinking will lead us. To start with, we can certainly say this: the carbon- hydrogen bond is stretched but not entirely broken, and the hydrogen-halogen bond has started to form but is not yet complete. This condition could be represented as.

Now, what can we say about the shape of the methyl group in this transition state? In the reactant, where methyl holds the hydrogen, carbon is tetrahedral (sp 3 hiybridized); in the product, where methyl has lost the hydrogen, carbon is trigonal (,sp2-hybridized). In the transition state, where the carbon-hydrogen bond is partly broken, hybridization of carbon is somewhere between sp3 and sp2. The methyl group is partly but not completely flattened; bond angles are greater than but less than 120.

Alkanes and Alkyl group: Isomers Alkanes are often described as saturated hydrocarbons: hydrocarbons because they contain only carbon and hydrogen atoms; saturated because they have only C-C and C-H single bonds and thus contain the maximum possible number of hydrogens per carbon. They have the general formula CnH 2 n+ 2, where n is any integer. Alkanes are also occasionally called aliphatic compounds, a word derived from the Greek aleiphas, meaning “fat.”

Think about the ways that carbon and hydrogen might combine to make alkanes. With one carbon and four hydrogens, only one structure is possible: methane, CH4. Similarly, there is only one possible combination of two carbons with six hydrogens (ethane, CH 3 CH 3 ) and only one possible combination of three carbons with eight hydrogens (propane, CH 3 CH 2 CH 3 ). If larger numbers of carbons and hydrogens combine, more than one kind of molecule can form. For example, there are two ways that molecules with the formula C 4 H 10 can form: the four carbons can be in a row (butane), or they can be branched (isobutane). Similarly, there are three ways in which C 5 H 12 molecules

Compounds like butane, whose carbons are connected in a row, are called straight- chain alkanes, or normal (n) alkanes, whereas compounds with branched carbon chains, such as isobutane (2-methylpropane), are called branched chain alkanes.

Compounds like the two C 4 H 10 molecules and the three C 5 H 12 molecules, which have the same formula but different structures, are called isomers, from the Greek isos _ meros, meaning “made of the same parts.” Isomers have the same numbers and kinds of atoms but differ in the way the atoms are arranged. Compounds like butane and isobutane, whose atoms are connected differently, are called constitutional isomers. Straight-chain alkanes are named according to the number of carbon atoms they contain, With the exception of the first four compounds—methane, ethane, propane, and butane—whose names have historical origins, the alkanes are named based on Greek numbers, according to the number of carbons. The suffix -ane is added to the end of each name to identify the molecule as an alkane. Thus, pentane is the five-carbon alkane, hexane is the six-carbon alkane, and so on.

Alkyl group If a hydrogen atom is removed from an alkane, the partial structure that remains is called an alkyl group. Alkyl groups are named by replacing the - ane ending with an -yl ending. For example, removal of a hydrogen atom from methane, CH 4, generates a methyl group, -CH 3, and removal of a hydrogen atom from ethane, CH 3 CH 3, generates an ethyl group, -CH 2 CH 3. Similarly, removal of a hydrogen atom from the end carbon of any n-alkane gives the series of n-alkyl groups shown below: