Week 6 notes CHEM

Transcription

1 Week 6 notes EM Unless otherwise stated, all images in this file have been reproduced from: Blackman, Bottle, Schmid, Mocerino and Wille, hemistry, 2007 (John Wiley) ISBN:

2 Note difference in conditions for Alkyl halide reactions Elimination eaction or Substitution? elimination Br hot conc. K in ethanol (solvent) KBr substitution Br dilute aq. Na + NaBr

3 Aldehydes and Ketones Aldehydes and ketones contain the carbonyl group i.e., =, a carbon-oxygen double bond arbonyl group 1 Aldehyde ( 1 = or 'carbon group') 1 2 Ketone ( 1 & 2 both 'carbon groups') In an aldehyde, = is bonded on one side to an, and on the other to a (or another ) In a ketone, a = is bonded on both sides to carbons

4 The arbonyl Group Very important chemical group, widely occurring in nature Lemons aspberries Important in spectroscopy: strong signals at about 1700 cm -1 associated with = in I Spectroscopy help us identify molecules

5 arbonyl compounds Ketones and aldehydes Double bond is composed of one σ and one π bond Electron density of π bond is above and below the plane of the double bond = double bond is polarised Absorption ~1700 cm -1 in infrared spectrum!! all four atoms lie in one plane

6 arbonyl compounds Ketones and aldehydes Blackman Figure 20.1

7 Bonding emember alkenes: = is 1 σ bond + 1 π bond Same for =, except that an oxygen replaces one carbon * 4 electrons shared * 2 in a! bond * 2 in a " bond * 4 electrons shared * 2 in a! bond * 2 in a " bond Both and sp 2 hybridised first bond (σ bond) formed by overlap of two sp 2 orbitals second bond (π bond) from overlap of two p orbitals

8 Bonding But two main differences from = bond 1. The oxygen retains two lone pairs Two lone pairs on 2. and don t share the electrons of the bond evenly ie. the bond is polarised Electronegativities:

9 arbonyl ompounds - Nomenclature ules follow guidelines for hydrocarbons: Find longest carbon chain containing the aldehyde or ketone group Ketone has two substituents attached to =; ending = -one Aldehyde has at least one attached to =; ending -al Number to give ketone or aldehyde lowest number Use di-, tri- where appropriate Ketone/aldehyde has preference over double bond (i.e., lower number) Examples: acetone (propan-2-one) * * acetaldehyde (ethanal) 5-hepten-2-one 3-methylbutanal * common usage name

10 Nomenclature More examples butanone propanal 3-hexanone 2-ethylpentanal

11 arbonyl ompounds - Nomenclature Question: Identify the following compounds as aldehydes or ketones and give their names

12 arbonyl ompounds - xidation ecall that when primary alcohols are oxidised, the final product is a carboxylic acid, but the intermediate is an aldehyde r / r / primary alcohol aldehyde carboxylic acid Aldehydes (but not ketones) are oxidised to give carboxylic acids upon treatment with acidic dichromate (r / + ) Aldehyde r / Ketone r / No eaction

13 arbonyl ompounds - xidation Give the products of the following reactions. (If no reaction would occur write N.) Na 2 r 2 7 / + Na 2 r 2 7 / + Na 2 r 2 7 / +

14 Nucleophilic Addition & Mechanism Polarity of = directs nucleophilic addition reaction The most important reaction of the carbonyl group Step 1: nucleophile uses its electron pair to attack at carbon Nu Nu Step 2: alkoxide anion reacts with positive portion (often + ) Nu Nu

15 Powerful nucleophiles are required to add to aldehydes or ketones : e.g., hydride ion ( - ) nucleophile use LiAl 4 or NaB 4 Na B Li Al

16 arbonyl ompounds - eduction Aldehydes and ketones are reduced by hydride ion ( - ), a nucleophilic reducing agent two-step addition Second adds as + NaB 4 Et 2 Et o alcohol First adds as Ketones react similarly, giving secondary alcohols

17 ydride eduction Use (LiAl 4 or NaB 4 ) followed by + Aldehydes primary alcohols, ketones secondary alcohols primary alcohol!! B secondary alcohol!! B

18 arbonyl ompounds - eduction The reduction reaction occurs via nucleophilic addition of - to the of the carbonyl group, followed by reaction of the alkoxide anion that is formed with + aldehyde 1. NaB 4 or LiAl 4 ; 2. 3 primary alcohol acetaldehyde hydride ion from NaB 4 or LiAl 4 3!! 3 STEP 1 STEP 2 nucleophilic acid-base addition reaction an alkoxide anion 3 a primary alcohol

19 arbonyl ompounds - eduction The mechanism is the same for aldehydes and ketones: ketone 1. NaB 4 or LiAl 4 ; 2. 3 secondary alcohol acetone hydride ion from NaB 4 or LiAl 4 3!! 3 3 STEP 1 STEP 2 nucleophilic 3 acid-base addition reaction an alkoxide anion 3 3 a secondary alcohol

20 arbonyl ompounds - eduction Give the products of the following reactions: 1. LiAl 4 ; LiAl 4 ; NaB 4 ; 2. 3

21 arboxylic Acids - Structure This functional group has different properties to either the ketone (--') group or the alcohol group (-) = group is polarised # = group of carboxylic acid contains one!-bond and one "-bond = alkyl, aryl, # # # bond is easily broken to give a proton and the carboxylate anion

22 arboxylic Acids - Nomenclature The longest chain containing the group gives the stem; ending -oic acid If substituents are present, start the numbering from the carboxyl group - 1 The conjugate bases are named by replacing the -oic acid ending with -oate Some simple carboxylic acids have non-systematic names arboxylic acid arboxylate anion Acyl substituent 3 acetic acid 3 acetate (ethanoate) 3 acetyl (ethanoyl) butyric acid (butanoic acid) (butanoyl) benzoic acid benzoate benzoyl

23 arboxylic Acids - Nomenclature Name the following carboxylic acids: Br

24 arboxylic Acids - Acid-Base eactions ecall that organic molecules containing - bonds (alcohols, phenols and carboxylic acids)can act as acids (proton donors) The carboxylate anion (the conjugate base) is resonance stabilised + carboxylic acid carboxylate anion: negative charge shared equally between two electronegative oxygen atoms

25 arboxylic Acids - Acid-Base eactions arboxylic acids react with dilute base (e.g. Na or Na 3 ) to form a water soluble carboxylate ion acid - base + Na reaction Na + 2 benzoic acid soluble in diethyl ether sodium benzoate insoluble in diethyl ether (soluble in Na solution) The carboxylate anion is a base (proton acceptor) and so reacts with dilute acid to give the corresponding carboxylic acid + l acid - base reaction + l

26 arboxylic Acids - eduction eactions ecall that oxidation of primary alcohols gives carboxylic acids as the product r 2 2 r / 7 / intermediate - not isolated The reverse reaction (i.e. reduction of carboxylic acids to give primary alcohols) requires loss of and addition of 2 x to the carboxylic acid This can be performed using LiAl 4 followed by 3 + LiAl 4 then 3 +

27 arboxylic Acids - eduction eactions Example: 3-propylhexanoic acid 1) LiAl 4 2) 3-propylhexanol Question: Give the product of each of the following reactions 1) LiAl 4 ; 2) 3 + 1) LiAl 4 ; 2) 3 +

28 arboxylic Acids - Substitution eactions Nucleophiles can react at the of the carbonyl group resulting in substitution reactions This is called nucleophilic acyl substitution eaction with thionyl chloride (Sl 2 ) gives acid chlorides + l S l 3 l + l + S acid chloride sulfur dioxide Example: Sl 2 l + l + S 2 benzoic acid benzoyl chloride

29 arboxylic Acids - eactions Draw the products of the following reactions: 1) LiAl 4 2) + Sl 2

30 arboxylic Acids - eactions Give the reagents required for the following reactions: l