9/20/2010. Functional Groups. Functional Groups. Functional Groups

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1 Functional Groups Functional group A given functional group behaves in nearly the same way in every molecule it is a part of The chemistry of every organic molecule, regardless of size and complexity, is determined by the functional groups it contains In the reactions of ethylene and menthene with bromine both molecules react with Br 2 in the same way Functional Groups Functional Groups with Carbon-Carbon Multiple Bonds Alkenes have double bonds Alkynes have triple bonds Arenes have alternating double and single bonds in a six-membered carbon ring The structural similarities in these compounds lead to chemical similarities Functional Groups Functional Groups with Carbon Singly Bonded to an Electronegative Atom Example functional groups include alkyl halides (haloalkanes), alcohols, ethers, alkyl phosphates, amines, thiols, sulfides and disulfides Bonds are polar Carbon bears a partial positive charge (d+) Electronegative atom bears a partial negative charge (d-) 1

2 Functional Groups Functional Groups with a Carbon-Oxygen Double Bond (Carbonyl Groups) Carbonyl group C=O Carbonyl carbon bears a partial positive charge (d+) Carbonyl oxygen bears a partial negative charge (d-) Functional Groups Functional Groups with a Carbon-Oxygen Double Bond (Carbonyl Groups) Present in organic compounds and in biological molecules Aldehydes Ketones Carboxylic acids Esters Thioesters Amides Acid chlorides 3.2 Alkanes are the simplest family of molecules Alkane A compound of carbon and hydrogen that contains only carboncarbon single bonds from the s-overlap of sp 3 hybrid orbitals General formula C n H 2n+2 where n is an integer 2

3 Alkanes often described as saturated hydrocarbons Hydrocarbons Contain only carbon and hydrogen Saturated Contain maximum possible number of hydrogens per carbon and have only C-C and C-H single bonds Alkanes occasionally referred to as aliphatic compounds, a name derived from the Greek word aleiphas, meaning fat Structures of representative alkanes Straight-chain alkanes or normal alkanes Alkanes whose carbon atoms are connected without branching Branched-chain alkanes Alkanes that contain a branching connection of carbons as opposed to a straight-chain alkane 3

4 Isomers From the Greek isos + meros meaning made of the same parts Compounds that have the same molecular formula but different structures Constitutional isomers Isomers that have their atoms connected in a different order May have different carbon skeletons, different functional groups, or different position of the functional groups A given alkane can be drawn arbitrarily in many ways Representations of butane n-c 4 H 10 where n denotes normal (straight-chain) These representations indicate only the connections among atoms These representations do not imply any particular threedimensional geometry Chemists rarely draw all the bonds in a molecule and usually refer to the condensed structure 4

5 Names of straight-chain alkanes Named according to the number of carbon atoms in the molecule, with the exception of the first four Names based on Greek numbers Suffix ane added to identify the molecule as alkane Worked Example 3.1 Drawing the Structures of Isomers Propose structures for two isomers with the formula C 2 H 7 N. 3.3 Alkyl Groups Alkyl group The partial structure that remains when a hydrogen atom is removed from an alkane Not stable compounds For naming of alkyl groups replace ane with yl ending Methane CH 4 methyl CH 3 Ethane CH 3 CH 3 ethyl CH 2 CH 3 5

6 Alkyl Groups Combining alkyl groups with various functional groups generates thousands of compounds Straight chains are generated by removing a hydrogen from an end carbon Alkyl Groups Branched alkyl groups are generated by removing a hydrogen atom from an internal carbon Alkyl Groups Prefixes for C 4 alkyl groups Refer to the number of other carbon atoms attached to the branching carbon atom Four possibilities Primary (1º) Secondary( 2º) Tertiary (3º) Quaternary (4º) R represents a generalized organic group 6

7 Alkyl Groups The terms primary, secondary, tertiary, and quaternary are routinely used to identify structural features Citric acid is a tertiary alcohol Citric acid has an alcohol functional group OH bonded to a carbon atom that is itself bonded to three other carbon atoms Alkyl Groups Hydrogen atoms Primary Attached to primary carbons (RCH 3 ) Secondary Attached to secondary carbons (R 2 CH 2 ) Tertiary Attached to tertiary carbons (R 3 CH) 3.4 Systematic nomenclature devised by the International Union of Pure and Applied Chemistry (IUPAC) Name has four parts 1. Prefix which specifies the location of functional groups and other substituents in the molecule 2. Parent selects the main part of the molecule and tells the number of carbon atoms 3. Locant gives location of primary functional group 4. Suffix identifies functional group family it belongs to 7

8 Alkanes can be named by following four steps 1. Find the parent hydrocarbon a) Find the longest continuing chain of carbon atoms in the molecule b) If two different chains of equal length are present, choose the one with the larger number of branch points as the parent 2. Number the atoms in the parent chain a) Number each carbon atom in the parent chain beginning at end nearer the first branch point b) If there is a branching point at an equal distance from both ends of the parent chain, begin numbering at the end nearer the second branch point 3. Identify and number the substituents a) Assign a number (called the locant) to each substituent to locate its point of attachment to the parent chain b) If there are two substituents on the same carbon, give them both the same number. There must be as many numbers in the name as there are substituents 8

9 4. Write the name as a single word If two or more different substituents are present write them in alphabetical order If two or more identical substituents are present, use the multiplier prefixes di-, tri-, tetra-, and so forth Do not use these prefixes when alphabetizing 5. Name a branched substituent as though it were itself a compound Begin numbering the branched substituent at its point of attachment to the main chain Substituent alphabetized according to the first letter of its complete name, including any numerical prefixes, and is set off in parentheses when naming the entire molecule More examples 9

10 Some alkanes have nonsystematic, common names 1. Three-carbon alkyl group 2. Four-carbon alkyl groups 3. Five-carbon alkyl groups Some compounds can be named with IUPAC rules or with common names Alphabetization in the naming of alkanes Nonhyphenated prefix iso- is considered part of the alkyl-group name when alphabetizing Isopropyl and isobutyl are listed alphabetically under i The hyphenated and italicized prefixes secand tert- are not considered part of the alkylgroup name when alphabetizing Sec-butyl and tert-butyl are listed alphabetically under b 10

11 Worked Example 3.2 Practice in What is the IUPAC name of the following alkane? Worked Example 3.3 Converting a Chemical Name into a Structure Draw the structure of 3-isopropyl-2-methylhexane. 11