CHEM 3013 ORGANIC CHEMISTRY I LECTURE NOTES CHAPTER 3

Transcription

1 EM 0 ORGANI EMISTRY I LETURE NOTES APTER Alkanes ydrocarbons contain only carbon and hydrogen Alkanes (also called araffins) are hydrocarbons which contain only single, sigma bonds between the carbons that make-u the backbone SP -S sigma bond normal bonds: in lane of age dark wedge: out of age dashed wedge: behind age SP -SP sigma bond Bonding in ethane 2 Normal alkanes Normal alkanes (n-alkanes) are linear, unbranched alkanes General Molecular Formula n2n+2 Members of a homologous series have redictable chemical and hysical roerties (m, b, density, reactivity) The first four straight chain alkanes have non-systematic names Thereafter, the alkanes are named using Greek number refixes (enta, hexa, heta, octa, nona, deca) followed by the suffix -ane methane ethane roane 2 butane ( 2 ) 2 entane ( 2 ) hexane ( 2 ) hetane ( 2 ) octane ( 2 ) nonane ( 2 ) decane ( 2 ) 8 general ( 2 ) n A homologous series with (2)n increasing by one methylene unit for each succeeding homolog n 2 n+2 The n-alkanes

2 Isomers 2 omounds with the same molecular formulas, but different structures are called isomers Isomers have different chemical and hysical roerties omounds which differ in their carbon connectivity are called structural isomers or constitutional isomers The number of ossible constitutional isomers increases dramatically as the number of carbon atoms increases Formula Number of isomers , ,9,8,80,8 Isomers of n-alkanes n-entane isoentane neoentane Non-systematic common names The structural isomers of 2 Alkyl grous If a hydrogen atom is removed form an alkane, the structure that remains is called an alkyl grou Alkyl grous are named by relacing the -ane ending of the arent alkane with -yl Removal of a hydrogen from methane generates a methyl grou, removal from ethane generates an ethyl grou Likewise, removal of a hydrogen from the terminal carbon of a straight-chain alkane generates a n-alkyl grou Removal of an internal hydrogen generates a branched alkyl grou

3 Methane methyl grou Ethane 2 ethyl grou Proane isoroyl grou royl grou Butane sec-butyl grou butyl grou Alkyl grous Alkane nomenclature The IUPA system divides a chemical name into three arts: refix, arent, and suffix The arent tells how many atoms are in the main chain, the suffix identifies the functional grous resent in the molecule, and the refix tells their osition along the main chain Stes for naming comlex, branched-chain alkanes i Select the longest chain of carbon atoms and give the comound the aroriate base name (See Table, age ) If two or more longest chains have the same number of carbons, select the one having the most grous ii Number the longest chain starting from the end nearest a branch iii Give all side chains a grou name by droing the "ane" of the name corresonding to the number of carbons in the side chain and add the new suffix of "yl" All side chains are numbered starting with the carbon attached to the base (longest) chain iv Precede each grou name with a refix which corresonds to the number of times the grou occurs along the chain (di, tri, tetra, enta, etc) v Place each grou name, including its refix, in front of the base name in alhabetical order Use the first letter of the grou name, but not the refix, to determine alhabetical order vi Place the number of the carbon, on which each grou is attached, before the grou name If a grou occurs more than once on a chain, all the locations of that grou must be laced before the grou name vii Searate numbers from numbers with commas and numbers from words with dashes Your final name will be one continuous word Naming omlex Grous omlex grous are side chains which have grous branching off the side chain i Find the longest chain in the grou, starting with the oint of attachment on the base chain ii Number the grou chain starting with the oint of attachment on the base chain

4 iii Name all grous along the grou chain according to the rules iii-vii above iv Enclose the entire grou name, in arenthesis v Precede the arenthetical statement with the number of the carbon, on the base chain, on which the grou is attached 2 -ethylhetane not longest chain and not lowest number 2 2-ethylhetane not longest chain 8 2 -methyloctane not lowest number 2 incorrect names 8 -methyloctane orrect Name Findingthe longest chain and lowest osition number 8 2 8,,,-tetramethyloctane Incorrect 2,,,-tetramethyloctane orrect! Lowest number to grous along the rincial chain arbon Tyes arbons in an organic comound are classified as to their structural tye in the molecule The structural tye is determined by how many other carbons the carbon under consideration is bonded to Primary carbons, ( ), are bonded to one other carbon Secondary carbons, (2 ) are bonded to two carbons Tertiary carbons,( ), are bonded to three others Quaternary carbons,( ), are bonded to four others

5 2 S t q t s 2 = rimary: ( bonded to ) s = secondary: ( bonded to 2 ) t = tertiary: ( bonded to ) q = quaternary: (bondedto ) arbon Tyes 8 ycloalkanes Alkanes formed into rings are called cycloalkanes, they have the general formula n2n As a short hand, cycloalkanes are reresented by olygons in skeletal drawings The smallest ring size ossible is a three membered ring called cycloroane Although large rings are ossible, the most common ring sizes are -,-,-,-, - and 8-membered rings Larger ring sizes are less common 9 ycloalkane Nomenclature ycloalkanes are named by rules similar to those used for alkanes The refix yclo- is used before the alkane name corresonding to the number of carbons in the ring For examle, consider a -membered ringa seven carbon alkane is hetanethus a -membered cycloalkane is cyclohetane Stes for naming substituted cycloalkanes i Use the cycloalkane name as the arent name, ie alkylcycloalkanes, unless an alkyl side chain has more carbons than the ringthen it becomes a cycloalkylalkane ii When more than one substituent is resent, start at a oint of attachment so as to give the lowest order iii When two or more different substituents are resent, number by alhabetical riority iv Treat halogens like alkyl grous

6 cyclohexane ethylcycloentane see rule i 2-cyclobutylhetane see rule i,2-dimethyl--ethylcyclohetane not -ethyl-,-dimethylcyclohetane see rule ii -ethyl-2-isobutylcyclobutane not -isobutyl-2-ethylcyclobutane see rule iii ycloalkane Nomenclature 0 is-trans Stereoisomerism in ycloalkanes The lack of flexibility in cycloalkanes (comlete rotation around - bonds is revented) has substantial consequences in that cycloalkanes have two sides, a "to" and a "bottom" side This leads to two distinct structures when the cycloalkane is substituted with various grous This is best illustrated in,2-dimethylcycloroane Note that the three carbons in cycloroane lie in the same lane (remember that you need three oints to define a lane) One can imagine one structure in which both methyls are on the same side of the ring lane, and another structure in which they are on oosite sides of the lane of the ring The connectivity of the methyls in the two different structures is the same owever the o rientation in sace of the two different methyls is different These two structures are STEREOISOMERS - comounds which have the same order of connectivi ty, but different three-dimensional geometry We define the two different threedimensional orientations by using the refix cis- (Latin for in the same side) or trans- (Latin; across)

7 Stereoisomers - same connections but different -D geometry cis-,2-dimethylcycloroane (same sides of rintg lane trans-,2-dimethylcycloroane (oosite sides of ring lane) an be reresented as trans-,2-dimethylcyclohexane Solid Wedge: (above lane) Dashed Wedge: (below lane) an be reresented as cis-,-dimethylcyclohexane Dashed Wedge: (below lane) Dashed Wedge: (below lane) is-trans Stereoisomerism