CHAPTER 12: Unit 2. Structures and shapes

Learning Objectives

1. Identify the general structure for an alcohol.
2. Identify the structural feature that classifies alcohols as primary, secondary, or tertiary.

Classifications of Alcohols: Alcohols re classified on the basis of the number of carbon atoms bonded to the carbon attached to the -OH group. In primary alcohol, the -OH group is bonded to a carbon atom which is bonded to only one C atom. In secondary alcohols, the -OH containing C atom is attached to two other C atoms. In tertiary alcohol, it is attached to three other C atoms.

Alcohols are classified as primary ( 1 0), secondary (20) and tertiary (3 0) depending on the number of carbon atoms bonded to the carbon atom that bears the hydroxyl group.

a primary alcohol is an alcohol in which the hydroxyl-bearing carbo atom is bonded to only one other carbon atom.

A secondary alcohol is an alcohol in which the hydroxyl bearing carbon atom is bonded to two other carbon atom.

A tertiary alcohol is an alcohol in which the hydroxyl-bearing carbon atom is bonded to three other carbon atoms.

An alcohol contains an oxygen atom surrounded by two atoms and alone pair of electrons, givingA bent shape like water. The bond angle is about 109.50Here are some basic IUPAC rules for naming alcohols:The longest continuous chain (LCC) of carbon atoms containing the OH group is taken as the parent compound—an alkane with the same number of carbon atoms. The chain is numbered from the end nearest the OH group.The number that indicates the position of the OH group is prefixed to the name of the parent hydrocarbon, and the –e ending of the parent alkane is replaced by the suffix –ol. (In cyclic alcohols, the carbon atom bearing the OH group is designated C1, but the 1 is not used in the name.) Substituents are named and numbered as in alkanes.If more than one OH group appears in the same molecule (polyhydroxy alcohols), suffixes such as –diol and –triol are used. In these cases, the –e ending of the parent alkane is retained. The names and structures of some alcohols demonstrate the use of IUPAC rulesExample 1Give the IUPAC name for each compound.HOCH2CH2CH2CH2CH2OHSolutionTen carbon atoms in the LCC makes the compound a derivative of decane (rule 1), and the OH on the third carbon atom makes it a 3-decanol (rule 2).The carbon atoms are numbered from the end closest to the OH group. That fixes the two methyl (CH3) groups at the sixth and eighth positions. The name is 6,8-dimethyl-3-decanol (not 3,5-dimethyl-8-decanol).Five carbon atoms in the LCC make the compound a derivative of pentane. Two OH groups on the first and fifth carbon atoms make the compound a diol and give the name 1,5-pentanediol (rule 3).Example 2Draw the structure for each compound.2-hexanol3-methyl-2-pentanolSolutionThe ending –ol indicates an alcohol (the OH functional group), and the hex– stem tells us that there are six carbon atoms in the LCC. We start by drawing a chain of six carbon atoms: –C–C–C–C–C–C–.The 2 indicates that the OH group is attached to the second carbon atom.Finally, we add enough hydrogen atoms to give each carbon atom four bonds.The ending –ol indicates an OH functional group, and the pent– stem tells us that there are five carbon atoms in the LCC. We start by drawing a chain of five carbon atoms:–C–C–C–C–C–The numbers indicate that there is a methyl (CH3) group on the third carbon atom and an OH group on the second carbon atom.Some of the common names reflect a compound’s classification as secondary (sec-) or tertiary (tert-). These designations are not used in the IUPAC nomenclature system for alcohols. Note that there are four butyl alcohols are shown below, corresponding to the four butyl groups: the butyl group (CH3CH2CH2CH2) Classification and Nomenclature of Some AlcoholsCondensed Structural FormulaClass of AlcoholCommon NameIUPAC NameCH3OH—methyl alcoholmethanolCH3CH2OHprimaryethyl alcoholethanolCH3CH2CH2OHprimarypropyl alcohol1-propanol(CH3)2CHOHsecondaryisopropyl alcohol2-propanolCH3CH2CH2CH2OHprimarybutyl alcohol1-butanolCH3CH2CHOHCH3secondarysec-butyl alcohol2-butanol(CH3)2(CH3)2CHCH2OHprimaryisobutyl alcohol2-methyl-1-propanol(CH3)3COHtertiarytert-butyl alcohol2-methyl-2-propanolsecondarycyclohexyl alcohocyclohexanolExercises1. Name each alcohol and classify it as primary, secondary, or tertiary.a.     CH3CH2CH2CH2CH2CH2OH b. c. 2. Name each alcohol and classify it as primary, secondary, or tertiary.a. b. c. 3. Draw the structure for each alcohol.a. 3-hexanolb. 3,3-dimethyl-2-butanolc. cyclobutanol 4. Draw the structure for each alcohol.a. cyclopentanolb. 4-methyl-2-hexanolc. 4,5-dimethyl-3-heptanolAnswers1.a. 1-hexanol; primaryb. 3-hexanol; secondaryc. 3,3-dibromo-2-methyl-2-butanol; tertiary 3.a. b. c.

Isomerism for alcohols: Constitutional isomerism is possible for alcohols containing three or more carbon atoms. Both skeletal and functional isomers are possible.Functional group Isomerism: Ethers and alcohols with the same number of carbon atoms and the same degree of saturation have the same molecular formula and thus isomers of each other. This type of constitutional isomerism is known as functional group isomerism.

In above diagram, alcoholic -Oh group is placed at a different positions in the chain giving rise to constitutional isomers. EthersLearning ObjectivesDescribe the structural difference between an alcohol and an etherWith the general formula ROR′, an ether may be considered a derivative of water in which both hydrogen atoms are replaced by alkyl or aryl groups. It may also be considered a derivative of an alcohol (ROH) in which the hydrogen atom of the OH group is been replaced by a second alkyl or aryl group:Simple ethers have simple common names, formed from the names of the groups attached to oxygen atom, followed by the generic name ether. For example, CH3–O–CH2CH2CH3 is methyl propyl ether. If both groups are the same, the group name should be preceded by the prefix di-, as in dimethyl ether (CH3–O–CH3) and diethyl ether CH3CH2–O–CH2CH3.