Organic Nomenclature

Transcription

1 University of Puget Sound Department of Chemistry Chem 111 Spring, 2010 Organic Nomenclature LEARNING GOALS AND ASSESSMENTS 1. Be familiar with the structure and nomenclature of organic compounds. a. Identify the characteristic functional group of each family of organic compounds. b. Given molecular formulas or drawings, supply the common or IUPAC name. c. Define conformation, and recognize eclipsed, staggered, anti, and gauche conformations. d. Define steric hindrance and use it to predict relative energies of conformations. e. Draw distinct constitutional isomers for a given alkane or alkene. f. Identify structural features conveyed by iso, s-, t-, 1, 2, 3, and 4. g. State the relationship between n and m in the formula C n H m, for alkanes, alkenes, and cycloalkanes. h. Identify and name distinct cis-trans stereoisomers. i. Define what is meant by mono- and di-substituted cycloalkanes. j. Describe how di-substituted cycloalkanes can have cis-trans stereoisomerism. k. Identify and name the designation (primary, etc.) of given amines. l. Sketch a carbonyl carbon. I. ALKANES A hydrocarbon is a compound that contains only hydrogen and carbon. Hydrocarbons containing only single bonds are called alkanes. The simplest member of the alkane family has one carbon bonded to four hydrogens. The name of this compound (CH 4 ) is obtained by putting together the root name for one carbon (meth) and the family name (-ane) to give methane. Other simple alkanes can be obtained by linking carbons together in a chain and adding hydrogens. Forms of an alkane that differ only by rotation about a single bond are referred to as conformations. Although the various conformations assumed by an alkane may look quite different, they are just different forms of the same molecule. This is because, for most alkanes at room temperature, rotation around single bonds is rapid; the energetic differences between them is usually quite small (a few tens of kj/mol). Usually, this energy difference can be accounted for on the basis of steric hindrance, i.e., atoms getting too close to each other. Constitutional isomers are different: to get from one constitutional form to another, you have to break a carbon-carbon bond, which normally does not happen at room temperature. The number of constitutional isomers goes up very quickly with the length of the molecule. Butane has only two constitutional isomers, but there are 366,319 isomers having the molecular formula C 20 H 42! 1

2 A hydrocarbon consisting of carbons and hydrogens in which the carbon atoms form a ring is called a cycloalkane, provided all the bonding is single bonds. Each ring is named according to the number of carbon atoms that makes up the ring. Substituted cycloalkanes occur when one of the hydrogens is replaced by a larger molecular fragment; when two substituents appear, it is called a di-substituted alkane. 1. Build a model of ethane using your molecular model set. Two conformations of ethane are the eclipsed and the staggered conformations. To see these, look down the end of the C C bond and rotate one methyl group relative to the other until the hydrogens line up; this is the eclipsed conformation. Now rotate the methyl group so that the hydrogens on the front are between the hydrogens on the back; this is the staggered conformation. The two conformations differ by 12.5 kj/mol. Q.1. Use steric hindrance to predict which of the two conformations of ethane has the highest energy. 2. Duplicate Table 1 in your lab notebook, and fill in the blanks. Use 3D models on the computer to assist you if necessary Table 1. Formulas and depictions of alkanes No. of Carbon Atoms Alkane Name Molecular Formula Condensed structural Formula Line Drawing 1 Methane CH4 CH4 (a dot) 2 Ethane C2H6 CH3CH3 3 Propane C3H8 4 Butane 5 Pentane 6 Hexane 3. Build a model of butane, and again observe the rotation of the C C bonds. By suitable rotations, construct the anti, eclipsed, and gauche conformations; reference to images on the computer will help. Q.2. Use steric hindrance to predict which conformation of butane has the highest energy. 4. Build a model of pentane this time, experiment with different constitutional isomers. It will help to leave off the hydrogens as you do this. 2

3 Q.3. Make line drawings of all distinct structural isomers of pentane. Q.4. For any hydrocarbon, the molecular formula can be represented as C n H m. Based on the pattern in methane, ethane, etc., derive a general formula relating m and n for any alkane. 5. Build a model of cyclobutane. Then, replace one of the hydrogens with the methyl group (-CH 3 ); this is called methylcyclobutane. Q.5. Make a line drawing of methylcyclobutane, and write its molecular formula. 6. Duplicate Table 2 in your notebook and fill in the missing values. Table 2. Formulas and depictions of cycloalkanes. No. of C Atoms in ring Cycloalkane Name Molecular Formula Line Drawing 3 Cyclopropane 4 Cyclobutane 5 Cyclopentane II. ALKENES The -ene suffix is given to compounds containing a carbon-carbon double bond. The position of the double bond is indicated by a number in front of the name that specifies where the double bond begins; in the case of two possible numbering schemes, we use the lower one. For example, if the double bond in a hexene molecule is between two carbons at the end, it is called 1-hexene, not 5-hexene. The position of the double bond leads to the possibility of more constitutional isomers compared to alkanes, but also to a different kind of isomerization called stereoisomerization. Stereoisomerization results from differences in spatial orientation. Cis-trans stereoisomers of alkenes result from different ways two substituents can be positioned on double-bonded (sp 2 ) carbons. 1. Build a model of trans 2-butene using your molecular set. It will help to open the Cis-trans butenes file in the Molecules folder. Note that the connectivity is the same for both molecules, and that the difference between the two is the spatial orientation of the methyl groups attached to the two sp 2 carbons. 2. Duplicate Table 3 in your lab notebook and fill in the blanks. 3

4 Table 3. Formulas and depictions of alkenes. IUPAC Name ethene 1-propene trans 2-butene cis 2-butene Molecular Formula Line Drawing Q.6. Describe (with the help of a drawing) how di-substituted cycloalkanes can have cis-trans stereoisomerism. III. AMINES Atomic grouping -NH 2 Suffix -amine Prefix amino Position in chain anywhere General formula C n H 2n+3 N We will use the common name system for amines. In addition, the designation primary, secondary, and tertiary amine refers to the number of carbon atoms connected to the nitrogen. 1. Build a model of methylamine using your molecular model set. 2. Duplicate Table 4 in your lab notebook and fill in the blanks. Table 4. Examples of Amines Common Name Line Drawing Designation (primary etc) Methylamine Dimethylamine Ethyldimethylamine 4

5 IV. ALCOHOLS Atomic grouping Suffix Prefix Position in chain General formula Common family name -OH -ol hydroxy anywhere C n H 2n+2 O alcohols 1. Build a model of ethanol using your molecular model set. 2. Duplicate Table 5 in your lab notebook and fill in the blanks. The primary, secondary, etc. designation is analogous to that of amines. Table 5. Structures of alcohols IUPAC Name Line Drawing Designation (primary etc) methanol ethanol 2-pentanol V. CARBOXYLIC ACIDS Atomic grouping -COOH Suffix -oic acid Prefix carboxy (includes the carbon) Position in chain only at end of chain General formula C n H 2n O 2 Common family name carboxylic acids The IUPAC name consists of two words. The carbonyl group in the alkanoic acid family is always at the end of a carbon chain and as such is always position 1 because of the bonding pattern R COOH. The carbon with the double bond to oxygen is known as the carbonyl carbon. 1. Build a model of methanoic acid using your molecular model set. 2. Duplicate Table 6 in your lab notebook and fill in the blanks. 5

6 Table 6. Examples of Carboxylic Acids IUPAC Name methanoic acid (a.k.a. formic acid) ethanoic acid (a.k.a. acetic acid) butanoic acid Line Drawing WHAT TO DO In your lab notebook: Write a title page, which includes the title, your name and your partner's name, the date, and your TA's name. Also lay out all the tables, one table per page. Check the text (e.g., Table 3.4) to make sure you understand the meaning of the terms molecular formula, condensed structural formula, and line drawing. During the lab: Complete the tables and answer the questions. To be Turned in at the End of Lab: Turn in the yellow pages that contain your completed tables and answers to all the questions (and you ll be done!). 6