Topic 6 Alkyl halide and carbonyl compounds Organic compounds containing a halogen

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1 Topic 6 Alkyl halide and carbonyl compounds rganic compounds containing a halogen ompounds are named in standard way, eg: iodo-2-methylpropane (tertiary alkyl halide) l 3 4-chlorotoluene (aryl halide) 1 5 l 3 trichloromethane (chloroform) l 4 tetrachloromethane (carbon tetrachloride) 2,4-dibromopentane (two secondary alkyl halides) 1 5 (2, 4)-2,4-dibromopentane (two secondary alkyl halides) Structure and properties Dense liquids and solids which are insoluble in water. The bond is polar with a slight positive charge () residing on the carbon end of the bond and a slight negative charge ( ) on the halogen end. = F, l, or The carbon halogen bond strength decreases in the order F > l > >. dipole moment Alkyl fluorides tend to be less reactive than other alkyl halides, mainly due to the higher strength of the F bond. Bond length (pm) Bond strength (kj mol 1 ) Dipole moment (D) 3 F l

2 1. cleophilic substitution Electrophile: A species that seeks an electron pair cleophile: A species that supplies an electron pair 3 3 nucleophile electrophile nucleophilic substitution Examples: 3 3 A wide range of charged and uncharged nucleophiles may be employed cleophile 3 Product Product class 3 3 alcohol 3 3 ether N 3 3 N nitrile 3 3 alkyne 2 N 3 3 N 2 amine 3 N 3 3 N 3 tetraalkylammonium salt Question: Draw the organic products in the following reactions: N N 2 41

3 chanism S N 2 (2 nd order) ~1/2 ~1/ nucleophile electrophile transition state leaving group S N 2 reaction favoured by a primary alkyl halide as the starting material. relative rate: methyl halide 2,000,000 (FAST) primary alkyl halide 40,000 relative rate: secondary alkyl halide 500 tertiary alkyl halide <1 (V. SLW) chanism S N 1 (1 st order) carbocations have sp 2 hybridised carbons (trigonlal planar) with a vacant p-orbital Attack of nucleophile to either side of the planar carbocation intermediate is equally likely! vacant p-orbital slow fast planar carbocation intermediate fast leaving group

4 Evidence for S N 1 mechanism ntermediacy of a planar carbocation intermediate means that any stereochemistry in the starting material is lost on conversion to the product. The nucleophilic substitution of ()-3-bromo-3-methylhexane with methanol leads to racemic methyl ether product 3 slow 3 ()-3-bromo-3-methylhexane rate determining step 50% retention planar carbocation intermediate 3 3 fast 50% inversion ()-3-methoxy-3-methylhexane S N 1 stands for substitution, nucleophilic, unimolecular. (S)-3-methoxy-3-methylhexane Unimolecular refers to the fact that only one molecule, the alkyl halide, is involved in the slow rate determining step. The reaction rate is therefore determined by the concentration of the alkyl halide eaction ate [electrophile] arbocations The intermediate in an S N 1 reaction is a carbocation. The atom has six electrons and a positive charge. This is an unstable, highly 3 reactive intermediate. 3 Not all carbocations have the 3 a tertiary alkyl same relative (un)stability. carbocation most stable 3 3 a secondary alkyl carbocation increasing stability 3 a primary alkyl carbocation a methyl carbocation least stable 3 S N 1 reaction more likely when a tertiary carbocation is the intermediate relative rate: methyl halide (V. SLW) <1 3 primary alkyl halide secondary alkyl halide relative rate: tertiary alkyl halide (FAST) 1,200,000 43

5 omparison of S N 1 and S N 2 Summary S N 2 reaction: Substitution, nucleophilic, second order The reaction rate is determined by the concentration of both nucleophile and electrophile nvolves the direct, single step substitution of an alkyl halide by a nucleophile with no intermediates The S N 2 reaction proceeds with inversion The rate of the S N 2 reaction of alkyl halides decreases in the order methyl > 1 > 2 >> 3 Usually only methyl, 1 and 2 alkyl halides react by this pathway - less crowded Summary S N 1 reaction: Substitution, nucleophilic, first order The reaction rate is determined by the concentration the electrophile only nvolves the two step substitution of an alkyl halide by a nucleophile via a carbocation intermediate The S N 1 reaction of chiral compounds proceeds with racemisation The rate of the S N 1 reaction of alkyl halides decreases in the order 3 >> 2 > 1 > methyl Usually only 3 alkyl halides react by this pathway Question: Fill in the missing reagents. N( 3 ) 3 Na 2 3 Na 3 Na 44

6 2. Elimination of Dehydrohalogenation require and on adjacent carbons. Generally concentrated reagents and elevated temperatures are used. oncentrated is used (often K in ethanol solvent). 3 2 c. heat 3 2 c. heat bromopropane 2 2-bromopropane Zaitsev s ule applies where there is a choice, the most substituted alkene forms. 3 l 2 c. heat 3 3 major product unsubstituted ethylene monosubstituted alkene disubstituted alkene trisubstituted alkene tetrasubstituted alkene Examples of eliminations hot, conc K ethanol 2 K hot, conc 2 S hot, conc K 2 l ethanol major trisubstituted = bond 3 carbon substituents K l minor disubstituted = bond 2 carbon substituents 45

7 Question: Fill in the reagents in the following sequence of reactions. 3. Formation of Grignard reagent This is the reaction of an alkyl or aryl halide and magnesium. Mg Mg = l,, = alkyl, aromatic eg 3 Mg 3 Mg Mg Mg l Mg Mgl 46

8 arbonyl ompounds aldehydes and ketones The double bond is composed of one σ and one π bond. The electron density of a π bond is above and below the plane of the double bond. The = double bond is polarised. Aldehydes have at least one attached to the carbonyl group, ketones have two carbon groups attached to the carbonyl group. arbon of the carbonyl group is sp 2 hybridised. overlap of electrons in p-orbitals give rise to a -bond oxygen has two lone pairs of electrons Nomenclature in formaldehyde, overlap of electrons in an sp 2 -hybridised orbital from and a 1s orbital from gives a -bond overlap of electrons in sp 2 -hybridised orbitals gives a -bond Aldehydes The longest chain containing the group gives the stem; ending al. f substituents are present, start the numbering from the aldehyde group 1. Ketones The longest chain containing the carbonyl group gives the stem; ending one. f substituents are present number from the end of the chain so the carbonyl group has the lowest possible number. There are non systematic names for the common aldehydes and ketone, for example: formaldehyde (methanal) acetaldehyde (ethanal) propionaldehyde (propanal) butyraldehyde (butanal) acetone (propanone) benzaldehyde 47 acetophenone

9 Question: Name the following compounds. 2 eactions of carbonyl group nucleophilic addition Dominated by weak and accessible π bond Polarity of = directs addition reaction eaction conducted in two stages cleophile (eg hydride or carbanion) adds to Then adds to stage 1 stage 2 1. ydride addition (reduction) Use (LiAl 4 or NaB 4 ) followed by Aldehydes give primary alcohols, ketones give secondary alcohols Examples: primary alcohol secondary alcohol 48

10 2. Grignard reaction nucleophile This is a very versatile reaction. an use most organohalide compounds to generate Grignard reagent. Grignard reagent reacts with aldehydes, ketones and 2. Must employ anhydrous conditions. use Grignard reagent: Mg Mg Mg Examples: 3 Mg methyl magnesium bromide formaldehyde 3 STEP 1 STEP 2 nucleophilic Mg acid-base addition an alkoxide reaction anion 3 a primary alcohol 3 Mg methyl magnesium bromide other aldehydes 3 Mg 3 secondary alcohol 3 Mg methyl magnesium bromide all ketones 3 Mg 3 tertiary alcohol 3 Mg 3 3 methyl magnesium bromide carbon dioxide Mg carboxylic acid 49

11 Question: Draw the products of the following sequence of reactions. l conc K heat dil 2 S 4 2 products, use one in next step r / conc 2 S 4 heat Mg 2. Question: n the Grignard reaction to form 3 methylhexan 3 ol there are three possible ketones that could be used as a starting material. dentify the three ketones and the Grignard reagent they need to react with to give the desired product. 50