P a g e 1 Chapter 12 INTRODUCTION TO ORGANIC CHEMISTRY: ALKANES Organic chemistry: The study of carbon compounds. Carbon is tetravalent; it always forms four bonds. Organic molecules have covalent bonds. Carbon forms multiple covalent bonds by sharing more than two electrons with a neighboring atom. In general: 1. A carbon that has 4 groups attached will be tetrahedral. 2. A carbon that has 3 groups attached will be trigonal planar. 3. A carbon that has 2 groups attached will be linear. When carbon bonds to a more electronegative element, polar covalent bonds result. Organic molecules have specific three-dimensional shapes. Organic molecules often contain nitrogen and oxygen in addition to carbon and hydrogen. Nature of Organic Molecules Most organic compounds are insoluble in water. Almost all of those that are soluble do not conduct electricity. Only small polar organic molecules or large molecules with many polar groups interact with water molecules and, thus, dissolve in water. Lack of water solubility for organic compounds has important consequences. Functional Groups Functional group: An atom or group of atoms within a molecule that has a characteristic physical and chemical behavior Organic compounds can be classified into families according to functional groups (structural features). The chemical behavior of family members is often predictable based on these specific groupings of atoms. Millions of compounds can be sorted into just a few general families of organic compounds with simple chemical patterns.
P a g e 2 A functional group is usually part of a larger molecule, and a molecule may have more than one class of functional group present. A given functional group tends to undergo the same types of reactions in every molecule that contains it. The chemistry of an organic molecule is primarily determined by the functional groups it contains, not by its size or complexity.
P a g e 3 Hydrocarbons The first four families are hydrocarbons, organic compounds that contain only carbon and hydrogen. Alkanes have only single bonds and contain no functional groups. Alkenes contain a carbon carbon double-bond functional group. Alkynes contain a carbon carbon triple-bond functional group. Simple aromatic compounds contain a six-membered ring of carbon atoms with three alternating double bonds. Alkyl Halides, Alcohols, Ethers, and Amines The next four families have functional groups that contain only single bonds and a carbon atom bonded to an electronegative atom. Alkyl halides have a carbon halogen bond. Alcohols have a carbon oxygen bond. Ethers have two carbons bonded to the same oxygen. Amines have a carbon nitrogen bond. Aldehydes, Ketones, Carboxylic Acids, Anhydrides, Esters, and Amides The next six families contain a carbon oxygen double bond: aldehydes, ketones, carboxylic acids, anhydrides, esters, and amides. Thiols, Sulfides, and Disulfides The remaining three families have functional groups that contain sulfur: thioalcohols (known simply as thiols), sulfides, and disulfides. These play an important role in protein function. Example 1 To which family of organic compounds do the following compounds belong? Explain.
P a g e 4 Example 2 Given the family of organic compounds to which the compound belongs, propose structures for compounds having the following chemical formulas: (a) An amine having the formula C2H7N (b) An alkyne having the formula C3H4 (c) An ether having the formula C4H10O Alkanes and Their Isomers Alkane: A hydrocarbon that has only single bonds. The general rule for all hydrocarbons except methane is that each carbon must be bonded to at least one other carbon. The carbon atoms bond together to form the backbone of the compound, with the hydrogens on the periphery. The general formula for alkanes is CnHn+2 where n is the number of carbons in the compound. Isomers As larger numbers of carbons and hydrogens combine, the ability to form isomers arises. Compounds that have the same molecular formula but different structural formulas are called isomers. For example, there are two ways in which molecules that have the formula C4H10 can be formed.
P a g e 5 Branching in Alkane Structures A straight-chain alkane is an alkane that has all its carbons connected in a row. A branched-chain alkane is an alkane that has a branching connection of carbons. Constitutional Isomers Constitutional isomers are compounds with the same molecular formula, but with different connections among their atoms. They are also known as structural isomers. Constitutional isomers of a given molecular formula are chemically distinct from one another. They have different structures and physical properties. When the molecular formula contains atoms other than carbon and hydrogen, the constitutional isomers obtained can also be functional group isomers. These are isomers that differ in both molecular connection and family classification. Example 3 Draw all isomers that have the formula C6H14.
P a g e 6 Condensed Structure Condensed structure: A shorthand way of drawing structures in which C C and C H bonds are understood rather than shown. Occasionally, not all the CH2 groups (called methylenes) are shown. CH2 is shown once in parentheses, with a subscript indicating the number of methylene units strung together. CH3CH2CH2CH2CH2CH3 = CH3(CH2)4CH3 Example 4 Write condensed structures for all isomers that have the formula C6H14. See example 3 above. Line Structure A line structure (or line-angle structure) is a shorthand way of drawing structures in which carbon and hydrogen atoms are not shown. Guidelines for drawing a molecule as a line structure: 1. Each carbon carbon bond is represented by a line. 2. Anywhere a line ends or begins, as well as any vertex where two lines meet, represents a carbon atom. 3. Any atom, other than another carbon or a hydrogen that is attached to a carbon must be shown. 4. Because a neutral carbon atom forms four bonds, all bonds not shown for any carbon are understood to be the number of carbon hydrogen bonds needed to have the carbon form four bonds. Example 5 Convert the following condensed structures to line structures:
P a g e 7 Example 6 Convert the following line structures to condensed structures: Example 7
P a g e 8 Example 8 Shapes of Organic Molecules Every carbon atom in an alkane has its four bonds pointing toward the four corners of a tetrahedron. The two parts of a molecule joined by a carbon carbon single bond in a noncyclic structure are free to spin around the bond, giving rise to an infinite number of possible conformations. The various conformations of a molecule are called conformers. Some conformations of butane (there are many others as well): At any given instant, most of the molecules have the least crowded, lowest-energy extended conformation. As long as two structures have identical connections between atoms, and are interconvertable, they are conformers. If two structures have the same name, they are conformers of the same compound. Newman Projections The Newman projection is the best way to judge the stability of the different conformations of a molecule.
P a g e 9 Ethane Conformations The torsional energy of ethane is lowest in the staggered conformation. The eclipsed conformation is about 3.0 kcal/mol (12.6 kj/mol) higher in energy. At room temperature, this barrier is easily overcome, and the molecules rotate constantly. Propane Conformations The staggered conformations of propane are lower in energy than the eclipsed conformations. Since the methyl group occupies more space than a hydrogen, the torsional strain will be 0.3 kcal/mol higher for propane than for ethane.
P a g e 10 Butane Conformations Butane has two different staggered conformations: gauche (60 between the methyl groups) and anti (180 between the methyl groups). The eclipsed conformation where the dihedral angle between the methyl groups is 0 is referred to as totally eclipsed.
P a g e 11 Steric Hindrance The totally eclipsed conformation is higher in energy because it forces the two end methyl groups so close together that their electron clouds experience a strong repulsion. This kind of interference between two bulky groups is called steric strain or steric hindrance. Example 9 The following structures all have the formula C7H16. Which of them represent the same molecule? Example 10 Are the following pairs of compounds the same (conformers), isomers, or unrelated?
P a g e 12 Example 11
P a g e 13 Naming Alkanes IUPAC Nomenclature The system of naming (nomenclature) was devised by the International Union of Pure and Applied Chemistry, IUPAC. In the IUPAC system for organic compounds, a chemical name has three parts: The prefix specifies the location of functional groups and other substituents. The parent tells how many carbon atoms are present in the longest continuous chain. The suffix identifies to which family the molecule belongs. Substituent: An atom or group of atoms attached to a parent compound Straight-chain alkanes are named by counting the number of carbon atoms and adding the family suffix -ane. Straight-chain alkanes have no substituents, so prefixes are not needed. Rule 1: The Main Chain Find the longest chain of consecutive carbons. The longest chain is six carbons: hexane When there are two longest chains of equal length, use the chain with the greatest number of substituents.
P a g e 14 Rule 2: Numbering the Main Chain Number the longest chain beginning at the end of the chain nearest a substituent. Rule 3: Name Alkyl Groups Substituents such as CH3 and CH2CH3, that branch off the main chain are called alkyl groups. An alkyl group is the part of the alkane that remains when a hydrogen atom is removed. Methyl group: CH3 Ethyl group: CH2CH3
P a g e 15 Applying Rules 1-3 i. Name the groups attached to the longest chain as alkyl groups. ii. Give the location of each alkyl group by the number of the mainchain carbon atom to which it is attached. iii. Write the alkyl groups in alphabetical order regardless of their position on the chain. 4-ethyl-2-methylhexane Multiple Groups When two or more of the same substituents are present, use the prefixes di-, tri-, tetra-, etc. to avoid having to name the alkyl group twice. Three methyl groups at positions 2, 5, and 7. 2,5,7-trimethyldecane Iso Groups Example 12 Give the structures of 4-isopropyloctane and 5-t-butyldecane. Rule 4: Organizing Multiple Groups
P a g e 16 Example 13 Give a systematic (IUPAC) name for the following compound. Example 14 What is the IUPAC name of the following alkanes? Example 15 Draw condensed and line structures corresponding to the following IUPAC names: (a) 2,3-Dimethylpentane (b) 3-Ethylheptane (c) 4-tert-Butylheptane (d) 2,2,4,4-tetramethylhexane (e) 2,3-dimethyl-4-propylnonane
P a g e 17 Cycloalkanes Cycloalkane: An alkane that contains a ring of carbon atoms To form a closed ring requires an additional C C bond and the loss of 2 H atoms. The general formula for cycloalkanes is CnH2n. Compounds of ring sizes from 3 through 30 and beyond have been prepared in the laboratory. The C C C bond angles in cyclopropane are 60, and the bond angles in cyclobutane are 90, much less than the ideal 109.5 tetrahedral angle. These compounds are less stable and more reactive than other cycloalkanes. The C C C bond angles in cyclopentane and cyclohexane are near ideal. Both cyclopentane and cyclohexane rings are stable, and many naturally occurring and biochemically active molecules, such as steroids, contain such rings. Properties of Cycloalkanes Because of their cyclic structures, cycloalkane molecules are more rigid and less flexible than their open-chain counterparts. Rotation is not possible around the carbon carbon bonds in cycloalkanes without breaking open the ring. This property is known as restricted rotation. This gives rise to geometric isomers. Cycloalkane Nomenclature Cycloalkane is the main chain; alkyl groups attached to the cycloalkane will be named as alkyl groups. If only one alkyl group is present, then no number is necessary.
P a g e 18 If there are two or more substituents, number the main chain to give all substituents the lowest possible number. The cycloalkane becomes a substituent when the acyclic portion of the molecule contains fewer carbons than the cyclic part or when there is a more important functional group in the molecule. Example 16 1. pentylcyclohexane 2. cyclobutylcyclohexane 3. cis-1-bromo-3-chlorocyclohexane 4. trans-1,3-diethylcyclopentane 5. isobutylcyclopentane 6. cis-1-ethyl-4-methylcyclohexane Properties of Alkanes Alkanes contain only nonpolar C C and C H bonds. The only intermolecular forces influencing them are weak London dispersion forces. The effect of London dispersion forces in alkanes is shown in the regularity with which the melting and boiling points of straight-chain alkanes increase with molecular size. The first four alkanes, methane, ethane, propane, and butane, are gases at room temperature and pressure. Alkanes with 5 15 carbon atoms are liquids. Alkanes with 16 or more carbon atoms are generally low-melting, waxy solids.
P a g e 19 As the number of carbons in an alkane increases, the boiling point increases due to the larger surface area and the increased van der Waals attractions. Melting points increase as the carbon chain increases. Alkanes with an even number of carbons have higher melting points than those with an odd number of carbons. Odorless or mild odor, colorless, tasteless, nontoxic. Nonpolar, insoluble in water but soluble in nonpolar organic solvents, less dense than water. Flammable, otherwise not very reactive. Reactions of Alkanes: Combustion and Halogenation Combustion The reaction of an alkane with oxygen is called combustion, an oxidation reaction that commonly takes place in a controlled manner in an engine or furnace. Carbon dioxide and water are the products of complete combustion of any hydrocarbon, and a large amount of heat is released. Halogenation Halogenation is the replacement of an alkane hydrogen by a chlorine or bromine initiated by heat or light. The process is known as free radical halogenation and occurs in a stepwise manner.
P a g e 20 Mechanism Free Radical Halogenation Many organic reactions yield a mixture of products. In a halogenation reaction, only one H at a time is replaced. If allowed to react for a long enough time, all Hs will be replaced with halogens. Complete chlorination of methane yields carbon tetrachloride: Example 17 (a) Draw all singly chlorinated isomers obtained upon the reaction of pentane with chlorine. (b) Draw all singly brominated isomers obtained upon the reaction of 2-methylbutane with bromine.