Outline for CH 231 at UA-Elem Organic Chem I (2)
Outline for CH 231 at UA-Elem Organic Chem I (2)
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Date Created: 02/06/15
Chapter 3 Outline 3 l Functional groups 0 Functional groups are groups of atoms bonded in a particular fashion that have a distinct chemical behavior Examples include I Carboncarbon multiple bonds I Carbon attached to one or more heteroatoms Each functional group will have a particular set of properties and reactivity As we learn about reactions of organic molecules the reactions will largely be classified by functional groups You should be able to recognize the following functional groups alkenes alkynes arenes halides alcohols ethers amines thiols sulfides nitriles nitro groups aldehydes ketones carboxylic acids esters and amides 32 Alkanes and alkane isomers o Alkanes are compounds made up of only carbon and hydrogen with all single bonds 0 Alkanes are part of the family of hydrocarbons compounds of carbon and hydrogen They are referred to as saturated hydrocarbons because they contain the maximum number of hydrogens Unsaturated compounds have one or more n bonds double or triple bonds To talk about alkanes and other organic compounds we need to know the language nomenclature used to name them I Organic compounds are named primarily based on the number of carbon atoms present I The family of alkanes are named methane 1C ethane 2C propane 3C butane 4C pentane 5C hexane 6C heptane 7C octane 8C nonane 9C decane 10C and so on see Table 33 I The iane suffix indicates that the molecule belongs to the alkane family The number of carbons is designated by the root name coming before the iane ending Thus hex is the root for 6 carbons and eth is the root for 2 carbons I Alkanes with 13 carbons can have only one possible arrangement With larger numbers of carbons other possible isomers can be formed I With 4 carbons there are 2 possible arrangements All 4 carbons can form a straight chain butane or a branched isomer can be formed isobutane The iso prefix indicates a branched structure with 2 CH3 groups attached to one carbon I Butane and isobutane are examples of constitutional isomers They have the same molecular formula but the atoms are bonded in different ways I With 5 carbons there are 3 possible structures pentane straight chain 2 methylbutane isopentane or 22dimethylpropane neopentane 39 As the number of carbons increases the number of possible structures grows exponentially I 33 Alkyl Groups 0 Alkanes are not very interesting by themselves They are more interesting when attached to functional groups 0 Alkyl groups are formed when one hydrogen is removed from an alkane to generate a site where another atom could be bonded o Alkyl groups are named by taking the name of the alkane ie ethane and replacing the 7 ane ending with a iyl ending Thus ethane becomes ethyl o In the case of propane there are two different kinds of hydrogens that can be removed 39 Removing a hydrogen from one of the CH3 groups gives a straight chain alkyl group called propane 39 Removing a hydrogen from the CH2 groups would result in a branched alkyl group This is called isopropyl Again it has 2 CH3 methyl groups attached to the central carbon 39 Butane similarly has two possible alkyl groups that can be formed butyl where the H is removed from a CH3 group and secbutyl where the H is removed from one of the CH2 groups 39 Isobutane provides 2 more C4 alkyl groups Removing an H from one of the CH3 groups gives the isobutyl group while removing the H from the CH group gives tert butyl 0 As we go along we ll often refer to carbons based on the number of other carbon atoms attached 39 Primary carbons are attached to one other carbon RCH3 where R is a generic organic group ie rest of the molecule 39 Secondary carbons are attached to 2 other carbons R2CH2 39 Tertiary carbons are attached to 3 other carbons R3CH 39 Quaternary carbons are attached 4 other carbons R4C 39 Note that the number of hydrogens does not matter Only the number of carbons Thus in CH3CH2OH the CH2 group is still a primary carbon and we could refer to this as a primary alcohol I 34 Naming alkanes 0 As you probably are starting to see there is a wide range of possible alkane structures We need a way to uniquely identify a molecular structure in words Therefore a systematic naming system has been developed The rules are established by the Union of Pure and Applied Chemists IUPAC 39 The goal of this naming convention is to have a name that provides all of the information necessary to completely describe the molecule To achieve this each name has to provide the number of carbons in the main chain the main functional group and any additional alkyl or functional groups attached The basic syntax is prefixlocantparentsuf x I Prefix provides number type and location of each substituent alkyl group or functional group I Locant provides the location of the main functional group if needed I Parent provides number of carbons in main chain alkane roots we39ve discussed I Suffix indicates the main functional group For examples alkanes have the iane suffix o How to name a compound Step 1 Find the parent hydrocarbon The number of carbons determined the parent name ie meth eth prop etc I The parent hydrocarbon is the longest continuous carbon chain I If more than one path has the same number of carbons choose the one with the most branches alkyl groups attached Step 2 Number the carbons in the main chain I Begin numbering on the end nearest the first branch point I If the first branch on each end are the same distance start on end nearest the second branch point I The overall goal is to give the substituents the lowest substituent numbers possible Step 3 Identify and number each substituent attached to the main chain I Alkyl groups are named as discussed in 33 I The number for the substituent is determined from the numbering of the main chain in step 2 I If there are 2 or more substituents on the same carbon each substituent is numbered Step 4 Write the name as a single word I Start with the substituents Each is named with the number location and the substituent name ie 4ethyl The number is separated from the name by a hyphen Put a hyphen on both sides if words are on both sides of the number 9 If there is more than one of a given substituent type ie 2 methyl groups use a number prefix di tri tetra penta etc to indicate how many are present Include the position number for each occurrence ie 22dimethyl or 246 tripropyl 9 If more than one substituent type is attached they are placed in alphabetical order The numerical pre xes are not used in determining the alphabetical order I Continue with the parent name and the suffix I Step 5 Complex substituents those with branches are named in the same way as molecules but replace the iane ending with yl I Determine the parent chain of the substituent Again this is the longest continuous carbon chain It must include the carbon attached to the main parent chain 9 Number the parent starting from the carbon attached to the main parent chain 9 Name the substituents on the side chain as normal I Include in the name as normal for a substituent but separate from the rest of the name with parentheses I 35 Properties of alkanes O Alkanes are generally unreactive They react with oxygen at high temperature to give C02 and H20 which is better known as combustion Because alkanes are nonpolar and few intermolecular attractions they have low boiling points I For example methane MW l6 boils at l6l 0C while water MW 18 boils at 100 0C I Branching causes lower boiling points and melting points I 36 Conformations of alkanes 0 If we consider the 3dimensional structure of ethane both carbons are tetrahedral we see there are two symmetrical structures we can draw I The CH bonds can be arranged opposite to each other on the two carbons This is called the staggered structure I The CH bonds can be aligned between the to methyl groups This is the eclipsed conformation I The geometries can be more easily shown using aNewman projection which shows what the molecule would like if looking down the CC bond axis Because there is relatively free rotation around single bonds the two conformers can rapidly interconvert I Conformational isomers conformers have identical attachment of the atoms but differ in their 3dimensional shape due to rotation about one or more bonds Generally conformers rapidly interconvert and cannot be separated The two conformations of ethane are not equal in energy The eclipsed conformer is 12 ldmole higher in energy than the staggered conformer I The higher energy is due to torsional strain in the eclipsed conformation Torsional strain results from the repulsion of electrons in the CH bonds When they are eclipsed this repulsion causes strain in the molecule raising its energy Therefore to rotate from one staggered conformer to another the molecule must pass through the 12 kJmole barrier of the eclipsed form This barrier is actually pretty small and bond rotation in ethane happens on the picosecond 103912 timefrarne at room temperature I 37 Conformations of other alkanes O Propane like ethane has both staggered and eclipsed conformations The eclipsed conformation is 14 ldmole higher in energy than the staggered Since we known eclipsed CiH bonds cause 4 kJmole of strain the CHCCH3 eclipsed strain must be 6 ldmol Butane has more possible conformations Consider Newman projections looking down the C2C3 bond One staggered conformation would have the CCH3 bonds pointing in opposite directions This is called the anticonformer and is the most stable conformation If we rotate 60 we have an eclipsed form where the CCH3 bonds are eclipsed with CH bonds The total strain is 664 l6 ldmole Rotating another 60 gives a new staggered conformation in which the CCH3 bonds are 60 apart from each other I Because of the size of the methyl groups there is some steric strain between them I Steric strain is due to groups bumping into each other due to their size I This conformation is called the gauche conformation and has a strain of 38 ldmole Rotating another 60 gives the least stable eclipsed conformation Here the two C H3 bonds are eclipsed with each other The total strain is 19 kJmole thus the eclipsing methyl groups have a strain of 11 kJmole This strain is a combination of steric and torsional strain Further rotation gives the same set of conformers as mirror images Higher alkanes have similar sets of conformations but with large alkyl groups the strains are larger
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