Alkanes.pdf

Alkanes Organic Chemistry Hydrocarbons Hydrocarbons are organic compounds contain only two elements, hydrogen and carbon. On the basis of structure, hydrocarbons are divided into two main classes, aliphatic and aromatic. Aliphatic hydrocarbons are further divided into families: alkanes, alkenes, alk...

Alkanes Organic Chemistry Hydrocarbons Hydrocarbons are organic compounds contain only two elements, hydrogen and carbon. On the basis of structure, hydrocarbons are divided into two main classes, aliphatic and aromatic. Aliphatic hydrocarbons are further divided into families: alkanes, alkenes, alkynes, and their cyclic analogs (cycloalkanes, etc.). Hydrocarbons Hydrocarbons Aliphatic hydrocarbons include three major groups: alkanes, alkenes, and alkynes. Alkanes are hydrocarbons in which all the bonds are single bonds, alkenes contain at least one carbon–carbon double bond, and alkynes contain at least one carbon–carbon triple bond. Examples of the three classes of aliphatic hydrocarbons are the two-carbon compounds ethane, ethylene, and acetylene. Hydrocarbons Another name for aromatic hydrocarbons is arenes. The most important aromatic hydrocarbon is benzene. Alkanes and Isomers Alkanes Alkanes have the general molecular formula CnH2n+2 Carbon will be sp3-hybridized when it is directly bonded to four atoms. All four sp3 orbitals are of equal energy. Each sp3 hybrid orbital has 25% s character and 75% p character. Alkanes Alkanes When they have a continuous chain of carbon atoms, and do not have any “branches”. They are called normal alkanes. Their structures are often drawn with the carbon chain in a horizontal line. Each name has the suffix -ane, which identifies the compound as an alkane. CH3—CH2—CH2—CH2—CH2—CH2—CH2—CH3 octane (a normal alkanc) Isomers Isomers are compounds that contain exactly the same number of atoms, i.e., they have exactly the same empirical formula, but differ from each other by the way in which the atoms are arranged. Constitutional isomers, compounds with the same molecular formula but a different connectivity of their atoms. Isomers Isopentane and neopentane are isomers of pentane. Isopentane is a branched alkane with four carbon atoms in the main chain and one branching methyl group. Neopentane has three carbon atoms in the main chain and two methyl groups bonded to the central carbon. Both normal and branched alkanes have the general molecular formula CnH2n+2. Classification of Carbon Atoms Hydrocarbon structures are classified according to the number of carbon atoms directly bonded to a specific carbon atom. A carbon atom bonded to only one other carbon atom is a primary carbon atom, which is designated by the symbol 1o. The carbon atom at each end of a carbon chain is primary. For example, butane has two primary carbon atoms. Classification of Carbon Atoms A carbon atom that is bonded to two other carbon atoms is a secondary carbon atom, designated by the symbol 2o. For example, the middle carbon atoms of butane are secondary. A carbon atom bonded to three other carbon atoms is tertiary, and is designated by 3o. A quaternary carbon atom (4o) is bonded to four other carbon atoms. Problem Pentaerythritol tetranitrate is a drug used to reduce the frequency and severity of angina attacks. Classify the carbon atoms in this compound. Names of Alkyl Groups Names of Alkyl Groups An alkane that has “lost” one hydrogen atom is called an alkyl group. Alkyl groups are named by replacing the –ane ending of alkane with -yl. The parent name of CH4 is methane. Thus, CH3– is a methyl group. The parent name of C2H6 is ethane, so CH3CH2– is an ethyl group. Names of Alkyl Groups Propyl and other primary alkyl groups derived from normal alkanes are normal alkyl groups. If the secondary carbon atom of propane loses a hydrogen atom, a secondary alkyl group known as the isopropyl group is formed. Names of Alkyl Groups Two alkyl groups are derived from butane and two from isobutane. Nomenclature of Alkanes Nomenclature of Alkanes IUPAC Rules for Naming Alkanes Alkanes and other organic compounds are named by the rules set forth by the International Union of Pure and Applied Chemistry (IUPAC). When these rules are followed, every chemical compound has a unique name. The IUPAC name consists of three parts: prefix, parent, and suffix. Nomenclature of Alkanes The IUPAC rules for naming alkanes are as follows: 1.The longest continuous chain of carbon atoms is the parent. If two possible parent chains have the same number of carbon atoms, the parent is the one with the larger number of branch points. Nomenclature of Alkanes 2. Number the carbon atoms in the longest continuous chain starting from the end of the chain nearer the first branch. The parent chains contains six carbons, so it is a hexane with two branches: a methyl group at C-2 and an ethyl group at C-4 Nomenclature of Alkanes If the first branch occurs at an equal distance from each end of the chain, number from the end that is nearer the second branch. The parent has eight carbons, so it is an octane. It has methyl groups at C-2 and C-7 and an ethyl group at C-4. Nomenclature of Alkanes 3. Each branch or substituent has a number that indicates its location on the parent chain. When two substituents are located on the same carbon atom, each must be assigned the same number. This octane has methyl groups at C-2, C-4, and C-6, and an ethyl group at C-4 Nomenclature of Alkanes 4. The number for the position of each alkyl group is placed immediately before the name of the group and is joined to the name by a hyphen. Alkyl groups and halogen atoms are listed in alphabetical order. The name of this compound is 4-ethyl-2methylhexane, not 2-methyl-4-ethylhexane Nomenclature of Alkanes Two or more groups of the same type are indicated by the prefixes di-, tri-, tetra-, and so forth. The numbers that indicate the locations of the branches are separated by commas. The name of this compound is 2,4-dimethylhexane Nomenclature of Alkanes 5. The prefixes di-, tri-, tetra-, and so forth do not alter the alphabetical ordering of the alkyl groups. The name of this compound is 5-ethyl-2,5dimethyloctane, not 3,5-dimethyl-5-ethyloctane Nomenclature of Alkanes Cycloalkanes with one ring have the general formula CnH2n compared with the general formula CnH(2n+2) for acyclic alkanes. Cycloalkanes are drawn as simple polygons in which the sides represent the carbon-carbon bonds. It is understood that each corner of the polygon is a carbon atom bonded to two hydrogen atoms. Physical Properties Physical Properties The boiling points and melting points rise as the number of carbons increases. The processes of boiling and melting require overcoming the intermolecular forces of a liquid and a solid; the boiling points and melting points rise because these intermolecular forces increase as the molecules get larger. Physical Properties The first four n-alkanes are gases, the next 13 (C5- C17) are liquids, and those- containing 18 carbons or more are solids. The alkanes are soluble in non-polar solvents such as benzene, ether, and chloroform, and are insoluble in water and other highly polar solvents. Considered themselves as solvents, the liquid alkanes dissolve compounds of low polarity and do not dissolve compounds of high polarity. Physical Properties Methane is the lowest boiling alkane, followed by ethane, then propane. All the alkanes with four carbons or fewer are gases at room temperature and atmospheric pressure. With the highest boiling point of the three, propane is the easiest one to liquefy. All of the carbon atoms have four bonds; all of the bonds are single bonds, and the bond angles are close to tetrahedral.