Chapter 19 part 2

Naming Organic Compounds

IUPAC System for Naming Organic Compounds The method recommended by the International Union of Pure and Applied Chemistry is systematic, generally unambiguous, and internationally accepted. Some trivial or common names are still used.

Alkyl Groups Alkyl groups have the general formula CnH2n+1 (one less hydrogen than the corresponding alkane). The name of the group is formed from the name of the corresponding alkane by simply dropping –ane and substituting a –yl ending. CH4 = methane -CH3 = methyl The letter R is often used in formulas to mean any of the many possible alkyl groups.

Table 19.3 Table 19.3 Names and Formulas of Selected Alkyl Groups

Primary (1°), Secondary (2°), and Tertiary (3°) Carbon Atoms

IUPAC Rules for Naming Alkanes 1. Select the longest continuous chain of carbon atoms as the parent compound, and consider all alkyl groups attached to it as branch chains that have replaced hydrogen atoms of the parent hydrocarbon. The name of the alkane consists of the name of the parent compound prefixed by the names of the branch-chain alkyl groups attached to it.

IUPAC Rules for Naming Alkanes 2. Number the carbon atoms in the parent carbon chain from one end to the other, starting at the end closest to the first carbon atom that has a branch chain. 3. Name each branch-chain alkyl group and designate its position on the parent carbon by a number.

IUPAC Rules for Naming Alkanes 4. When the same alkyl-group branch chain occurs more than once, indicate this by a prefix (di-, tri-, tetra-, etc.) written in front of the alkyl group name (e.g. dimethyl indicates two methyl groups). The numbers indicating the positions of these alkyl groups are separated by a comma, followed by a hyphen, and placed in front of the name.

IUPAC Rules for Naming Alkanes 5. When several different alkyl groups are attached to the parent compound, list them in alphabetical order (e.g. ethyl before methyl as in 3-ethyl-4-methyloctane).

Name the following alkane:

Write the formula for 3-ethylpentane

Write the formula for 2,2,4-trimethylpentane

Introduction to the Reactions of Carbon

One of the most important carbon reactions is oxidation reduction.

Oxidation When carbon atoms are oxidized, they often form additional bonds to oxygen.

Reduction When carbon atoms are reduced, they often form additional bonds to hydrogen.

Other Reactions of Carbon Substitution reactions Elimination reactions 3.Addition reactions

Substitution Reaction If, in a reaction, one atom in a molecule is exchanged by another atom or group of atoms, a substitution reaction has taken place.

Elimination Reaction An elimination reaction is a reaction in which a single reactant is split into two products, and one of the products is eliminated. Elimination reactions form multiple bonds.

Addition Reaction Two reactants adding together to form a single product is called an addition reaction. An addition reaction can be thought of as the reverse of an elimination.

Reactions of Alkanes

Combustion of Alkanes Alkanes, as well as other hydrocarbons, undergo combustion with oxygen with the evolution of large amounts of heat energy. CH4(g) + 2O2 (g)  CO2(g) + 2H2O(g) + 802.5 kJ When CO2 is formed, the alkane has undergone complete oxidation.

Aside from their combustibility, alkanes are relatively sluggish and limited in reactivity. With proper activation, such as with high temperature or catalysts, alkanes can be made to react in a variety of ways.

Some Important Noncombustion Reactions of Alkanes Halogenation a substitution reaction Dehydrogenation an elimination reaction Cracking breaking up large molecules to form smaller ones Isomerization rearrangement of molecular structures

Some Important Noncombustion Reactions of Alkanes Halogenation (a substitution reaction) A halogen is substituted for a hydrogen atom. CH3CH3 + Cl2  CH3CH2Cl + HCl

Alkyl Halides The formula RX indicates a halogen atom attached to an alkyl group and represents the class of compounds known as alkyl halides. Ex: CH3CH2CH2Cl 1-chloropropane CH3CH2CH2CHBr2 1,1-dibromobutane

Some Important Noncombustion Reactions of Alkanes Dehydrogenation (an elimination reaction) Hydrogen is lost from an organic compound. CH3CH2CH3  CH3CH=CH2 + H2

Some Important Noncombustion Reactions of Alkanes Cracking C16H34  C8H18 + C8H16

Some Important Noncombustion Reactions of Alkanes Isomerization

Sources of Alkanes The two main sources of alkanes are natural gas and petroleum. Natural gas is formed by the anaerobic decay of plants and animals. Petroleum, also called crude oil, consists of a mixture of hydrocarbons and is formed by the decomposition of plants and animals over millions of years.

Gasoline: A Major Petroleum Product

Figure 19.9 Uses of petroleum

Cycloalkanes Cycloalkanes have the formula CnH2n. Their names are formed by adding the prefix cyclo to the name of the alkane with the same number of carbon atoms.

Cycloalkanes With the exception that follows for cyclopropane and cyclobutane, cycloalkanes are generally similar to open-chain alkanes in both physical properties and chemical reactivity. Cycloalkanes are saturated hydrocarbons; they contain only single bonds between carbon atoms.

Figure 19.11 Ball-and-stick models illustrating cyclopropane, hexane, and cyclohexane. In cyclopropane, all the carbon atoms are in one plane (bond angles = 60°). The cyclohexane molecule is puckered (as shown in the chair conformation) with bond angles at 109.5°.

Reactivity of Cyclopropane and Cyclobutane There is a greater reactivity in cyclopropane and cyclobutane because the carbon-carbon bond angles in these substances deviate substantially from the tetrahedral angle. Their carbon-carbon bonds are strained and therefore weakened.

Conformations of Cyclohexane The conformation of a molecule is its three-dimensional shape in space. Figure 19.12 Conformations of cyclohexane: (a) chair conformation; (b) boat conformation. Axial hydrogens are shown in blue in the chair conformation.

Naming Cycloalkanes Substituted cycloalkanes are named by identifying and numbering the location of the substituent(s) on the carbon ring, followed by the name of the parent alkane.

Naming Cycloalkanes When a cycloalkane has two or more substituted groups, the ring is numbered (clockwise or counterclockwise) to give the location of the substituted groups the smallest numbers.

Naming Cycloalkanes