Chem 401 Lab Exercise #5 Nomenclature Worksheet for Alkanes and Cycloalkanes

Transcription

1 hem 401 Lab Exercise #5 Nomenclature Worksheet for Alkanes and ycloalkanes Structure and Nomenclature of Alkanes Alkanes are saturated hydrocarbons; that is, they contain only and which are bonded solely with σ bonds. As the valence of is 4 and the valence of is 1, will have 4 bonds and may only have 1. Because following this rule ( will get 8 electrons or 4 bonds; will get 2 electrons or 1 bond), we don t bother counting valence electrons. Alkanes have the general formula n 2n+2. The simplest alkanes are continuous chain alkanes. They are also called straight chain alkanes, although the chain is not really straight, it zig-zags! In continuous chain alkanes, the atoms are connected together in a line, and the atoms fill around. ere are the structural formulas for the 3 simplest continuous chain alkanes: Line Structure or Kekulé Structure ondensed Structure 4 or or Skeletal Structure N/A Note that there is more than one way to draw or write the structure. The Kekulé structure would be like the Lewis structure. The condensed structure doesn t show all the bonds: the line bonds between and are omitted; and even the line bonds between atoms may be dropped. Instead, due to the predictability of and bonds, the - and - bonds are understood. Notice that in the condensed structure, the atoms immediately follow the to which they are attached. In the skeletal structure, the atoms are omitted altogether, and the atoms are implied wherever a line ends or at the intersection of 2 lines. The reader can easily determine how many atoms are attached to each as the total number of bonds must equal Draw the Kekulé, condensed, and skeletal structure for the continuous chain alkane with the formula What is the molecular formula for the following skeletal structure? In naming straight chain alkanes, we combine a parent root with the suffix -ane. The parent root and names for the most common alkanes are below. You should memorize these parent root and names. The parent root for 5 and above come from the Greek or Latin names for numbers, while the parent root for 1 to 4 are common names.

2 Table: Parent Names of ontinuous (Straight) hain Alkanes # of atoms Parent Root Name ondensed Structure 1 meth- methane 4 2 eth- ethane 3 prop- propane 4 but- butane 5 pent- pentane 6 hex- hexane 7 hept- heptane 8 oct- octane 9 non- nonane 10 dec- decane 11 undec- undecane 12 dodec- dodecane 20 ico- icosane ( ) Write the Kekulé structure for decane. 4. Name the following alkanes: a. b. ycloalkanes yclic alkanes are hydrocarbon chains where two end atoms join to form a ring. Because of this extra - bond, two atoms are lost in simple cycloalkanes. So the general formula is n 2n. To name simple cycloalkanes, we add the prefix cyclo to the parent name for the corresponding continuous chain alkane. So cyclopropane is: 5. Draw the structure for cyclohexane.

3 Alkyl Groups and Branched Alkanes There are many alkanes which do not have a simple straight chain structure. Instead, they are what we call branched alkanes. In branched alkanes, 1 or more atoms are substituted with alkyl groups or alkyl substituents. Branched alkanes are named after the parent name for the longest continuous chain alkane present in the molecule. For example, consider the following structure: 6. Draw a line through the longest continuous chain of atoms. 7. ow many atoms are in the longest continuous chain (main chain)? 8. What is the parent name for the main chain? 9. ow many branches or alkyl groups are there on the main chain? ow do you name a more complex branched alkane when it has alkyl (or cycloalkyl) groups? The alkyl group substituents are named by taking the parent root and and adding the suffix -yl. All chemists recognize that the -yl ending means that this is an alkyl substituent. See the following Table to see the trend. nce the alkyl groups are identified, they are added as prefixes to the parent alkane name. You will learn how in the following section. Table: A Few ommon Alkyl Groups # of Parent Root Alkyl Group ondensed Structure atoms Name 1 meth- methyl - 2 eth- ethyl - 3 prop- propyl - 4 but- butyl 5 pent- - 6 hex- 4 cyclobut- cyclobutyl 6 cyclohex- cyclohexyl 10. Fill in the blanks in the above table.

4 11. In the above structure (used in 6-10), what are the names of the 2 alkyl groups? and. Use the following structure for Q ow many atoms are in the longest continuous chain (main chain)? 13. What is the parent name for the main chain? 14. ow many branches or alkyl groups are there on the main chain? 15. The names for the alkyl groups are. Rules for Naming Branched Alkanes (and yclalkanes) 1. Find the longest continuous chain. This is the main or parent chain. If there is a ring with more atoms than the longest continuous chain, it is considered the parent chain. If the ring contains fewer than the longest continuous chain, then the ring is treated as a cycloalkyl group. If 2 or more chains of equal length may be identified, the parent chain should be the chain containing more branches. Examples: Longest continuous chain is 11 : so parent chain name is undecane. Note that the longest continuous chain is not necessarily in a straight line! Longest chain is 7 or heptane; there are 2 substituents; a branched ethyl group at position 3 and a methyl group at position 4. 3 yclooctane is parent name; methyl and propyl group are substituents pentane is parent name as it has more atoms than cyclobutane; cyclobutyl group is substituent Longest chain is 7 or heptane; there are 3 substituents; an ethyl group at position 3 and methyl groups at position 2 and 4. This would be the correct way to number this chain.

5 2. Number the atoms in the parent chain so that the branches or alkyl groups have the lowest numbers possible. When there are multiple alkyl groups, check to make sure that you have assigned the lowest possible numbers For this numbering system, the numbers for the substituents are 2, 3, and 4. This would be the correct way to number this chain For this numbering system, the numbers for the substituents are 4, 5, and 6. These are not the lowest numbers possible. 3. Number and name the substituents. Both the number and the name will be used in naming the compound. If there are more than 1 of the same substituent, a prefix such as di, tri, tetra, etc. is used before the name of the group. The numbers for the repeated groups would be separated by commas There are methyl groups at position 2 and 4, and there is an ethyl group at position 3. The prefix and name for the ethyl group will be 3-ethyl; while the prefix and name for the two methyl groups will be 2,4-dimethyl 4. Attach the number and name of the substituents to the name of the parent chain. Write the full compound name as a single name: use hyphens to separate numbers from substituents (i.e. 3-ethyl-2-methyl), and numbers are separated by commas (i.e. 2, 4-dimethyl). If there are multiple substituents, the substituent names are placed in alphabetical order, regardless of their numbered position (so ethyl comes before methyl). For alphabetizing, only the substituent name is considered, do not consider the multiplying prefixes di, tri, tetra, etc. These only tell you how many of this substituent there are, they are not the parent name of the group. (so 5-ethyl-2,2-dimethyl would be correct as ethyl comes before methyl; 2,2-dimethyl-5-ethyl would be incorrect) The prefixes and names will be 3-ethyl and 2,4-dimethyl. In this case, the di does not count towards alphabetizing as it is not part of the group name (group name is methyl, there are just 2 of them); so ethyl comes before methyl. The complete name is: 3-ethyl-2,4-dimethylheptane

6 Use the following structure for Q ow many atoms are in the longest continuous chain (main chain)? 17. What is the parent name for the main chain? 18. The name of the substituent is and its numbered position (give lowest possible number) is. 19. The number of the substituent is separated by a from its name, while there is no separation between the substituent name and the name of the parent chain. 20. The name for this compound is. 21. Draw a structure for the following compounds: a. 3-methylpentane b. 1-propylcyclopentane c. 4,5-diethyl-2-methyldodecane Name the following compounds

7 Isomers: Structural (onstitutional) Isomers and cis, trans Stereoisomers Two or more different compounds which have the same molecular formula are called isomers. Structural (also called constitutional) isomers are compounds in which the atoms are bonded in a different order. ere are the two structural isomers possible for the molecular formula Name the above isomers. 26. Draw and name all possible isomers of Draw and name all possible isomers of 5 10.

8 Another type of isomer are stereoisomers. Stereoisomers have the same molecular formula, and the atoms are connected in the same order (same bond connectivity), BUT have different orientations in space. So the atoms are arranged differently in space. There are several types of stereoisomers, but the simplest type is called cis-trans stereoisomers (also called geometric isomers). This type of stereoisomer occurs when there is a source of rigidity in the molecule. ycloalkanes and alkenes both exhibit cis-trans isomers. In cycloalkanes, the closed ring structure restricts rotation about the - bond, so the 2 substituents on each ring may point up or down. When we draw rings, we can show this up vs. down by drawing the connecting lines so that it is clear that one group is pointing up while the other group is pointing down. In order for cycloalkanes to show cis-trans isomerism, at least 2 ring atoms must have 2 different groups attached. Some examples follow. Two groups are cis Two groups are trans Dashed arrow goes into paper Solid arrow comes out of paper Two groups are cis as both point in same direction. Name is cis-1,2-dimethylcyclopentane. Notice cis is in italicized in name. Two groups are trans as one points up and the other points down. Name is trans-1,2-dimethylcyclopentane. No cis-trans isomerism as only 1 has 2 different groups. 28. Name the following compound (remember that when equal numbering systems are possible, give the lower numbers in alphabetical order.

9 29. Draw the correct structure for cis-1-methyl-4-propylcyclohexane. Alkenes contain at least 1 double bond. The π bond in the double bond restricts - rotation, so the two substituents on each of the double bond carbons are fixed in relation to each other. Substituents may be on the same side of the double bond (cis) or they may be on opposites sides of the double bond (trans). In order for alkenes to show cistrans isomerism, each atom involved in the double bond must have 2 different groups attached to it ( included) different groups on each double bond. The methyl groups are on the same side of the double bond, so they are cis. Name is cis-2-butene. Note that the longest chain is a but- chain with a double bond in the number 2 position. Therefore, this gives the 2-butene portion of the name. So going from an alkane to an alkene changes the suffix from -ane to -ene. ere the methyl groups are on opposite sides of the double bond, so they are trans. Name is trans-2-butene. 3 3 The first double bond has 2 methyl groups, so no isomerism possible. Name is 2-methyl-2-pentene. 30. Draw and label the cis and trans isomers of 3-hexene. (The double bond STARTS at the 3rd.) Alkyl alides alogens are a common alkane substituent. It is quite easy to name alkyl halides as you treat the halide group(s) exactly as you would treat an alkyl group. For example, in the following structure you treat the chloro group at position 2 the same as any alkyl group. So the name is 2-chloro-3-methylhexane. l 31. Draw the structure for 2-chloro-2,3-dimethylbutane. 32. Name the following compound. Br

10 ther Functional Groups Although there are many functional groups, there are several which are most important for you to know at this point. They are: alcohols, ethers, carboxylic acids, ketones, aldehydes, esters, amides, alkenes, and alkynes. Family Name Group Name Group Structure General Formula Name Ending alkane alkyl - R 3 *-R 3 * -ane alkene double bond; alkenyl = R 2 *=R 2 * -ene alkyne triple bond; alkynyl R* R* -yne alcohol hydroxyl - R -ol ether alkoxyl -R RR ether carboxylic acid carboxy - 2 R* 2 -oic acid ester alkoxycarbonyl - 2 R R* 2 R -oate ketone oxo -(=)R R(=)R -one aldehyde oxo -(=) R*(=) -al amide amido -(=)N 2 R*(=)N 2 -amide * ere the R s may be atoms or alkyl groups. If no asterisk, then the R must be an alkyl group and can t be. So when you go from an alkane to an alkene, the -ane ending becomes -ene. So the name ending basically tells you what the most important functional group is. (Many compounds contain more than 1 functional group, so the naming is more complicated. For now, you will only be expected to be able to name simple compounds.) For example, here are the structures and names for some simple compounds: propane propene propyne Do you see how the name changes as you go from a single to a double to a triple bond? Also, do you see that the alkyne drawn above correctly shows its linear geometry around the triple bond s? Remember, these triple bond atoms are sp hybridized, giving a linear geometry.

11 1-propanol 2-propanol ethyl methyl ether dimethyl ether In the above 4 structures, the first 2 are alcohols while the other 2 are ethers. An - group is the alcohol group, so the name becomes -ol. Also note, that for alcohols with a hydrocarbon chain of 3 or longer, the position of the - group must be specified. The ethers are more complex (don t worry about naming them now), but do you see the -- linkage? This is the hallmark of an ether group. propanal 2-propanone or just propanone Aldehydes get an -al ending and are very simple to recognize. They have the = group at the end of the alkyl chain so there is always an attached to the = carbon (this carbon is called the carbonyl carbon). In the above structure for propanal, the end is not shown, but if you count the bonds on the end, you should realize that there is 1 attached to it. Ketones are also simple to recognize with the R 2 = group. Now neither R group can be an as that would make it an aldehyde. So the difference between ketones and aldehydes is that aldehydes have the carbonyl on the end of a chain, while ketones have the carbonyl inside the chain. For ketones, this means that for chains of 5 or more carbons, the position of the = must be specified. For chains of 3 or 4, it may be specified as above, but it is not strictly necessary. propanoic acid N 2 propanamide methyl propanoate The carboxylic acids have a R(=) ending [the parenthesis here means that the = is attached to the right before this (=). The - group is also attached to the same, the carbonyl ]. So the carbonyl is on the end like an aldehyde, but the of an aldehyde becomes an - group. So an acid is on the end of a chain. Amides and esters are derived from carboxylic acids and can get more complicated to name (so don t worry now). Just be able to recognize the group. For simple amides, the - group of a carboxylic acid becomes an -N 2 group (these s can also be alkyl groups). For esters, the - group of a carboxylic acid becomes an -R group like - or -. This is basically an ether ending. This ether ending gets named first (for simple esters).

12 Draw the following compounds: 33. Ethanol 34. Ethanoic acid (acetic acid) 35. Butanal pentanone pentanone hexanol hexanol 40. Pentanal