Chapter 20 Carboxylic Acids. Introduction

Transcription

1 hapter 20 arboxylic Acids Introduction arbonyl (-=) and hydroxyl (-H) on the same carbon is carboxyl group. arboxyl group is usually written -H or 2 H. Aliphatic acids have an alkyl group bonded to -H. Aromatic acids have an aryl group. Fatty acids are long-chain aliphatic acids. hapter 20: arboxylic Acids Slide

2 ommon Names Many aliphatic acids have historical names. Positions of substituents on the chain are labeled with Greek letters. hapter 20: arboxylic Acids Slide 20-3 IUPA Names Remove -e from alkane (or alkene) name, add -oic acid. The carbon of the carboxyl group is #1. l H 3 H 2 H H 2-chlorobutanoic acid Ph H H H trans-3-phenyl-2-propenoic acid (cinnamic acid) hapter 20: arboxylic Acids Slide

3 Naming yclic Acids ycloalkanes bonded to -H are named as cycloalkanecarboxylic acids. Aromatic acids are named as benzoic acids. H H(H 3 ) 2 2-isopropylcyclopentanecarboxylic acid H H o-hydroxybenzoic acid (salicylic acid) hapter 20: arboxylic Acids Slide 20-5 Dicarboxylic Acids Aliphatic diacids are usually called by their common names (to be memorized). For IUPA name, number the chain from the end closest to a substituent. Two carboxyl groups on a benzene ring indicate a phthalic acid. H Br HH 2 HH 2 H 2 H 3-bromohexanedioic acid β-bromoadipic acid H 1,3-benzenedicarboxylic acid m-phthalic acid hapter 20: arboxylic Acids Slide

4 Structure of arboxyl arbon is sp 2 hybridized. Bond angles are close to H eclipsed with =, to get overlap of π orbital with orbital of lone pair on oxygen. hapter 20: arboxylic Acids Slide 20-7 Boiling Points Higher boiling points than similar alcohols, due to dimer formation. Acetic acid, b.p. 118 hapter 20: arboxylic Acids Slide

5 Melting Points Aliphatic acids with more than 8 carbons are solids at room temperature. Double bonds (especially cis) lower the melting point. Note these 18- acids: Stearic acid (saturated): 72 leic acid (one cis double bond): 16 Linoleic acid (two cis double bonds): -5 hapter 20: arboxylic Acids Slide 20-9 Solubility Water solubility decreases with the length of the carbon chain. Up to 4 carbons, acid is miscible in water. More soluble in alcohol. Also soluble in relatively nonpolar solvents like chloroform because it dissolves as a dimer. hapter 20: arboxylic Acids Slide

6 Acidity hapter 20: arboxylic Acids Slide Resonance Stabilization Important for anion hapter 20: arboxylic Acids Slide

7 Substituent Effects on Acidity hapter 20: arboxylic Acids Slide Salts of arboxylic Acids Sodium hydroxide removes a proton to form the salt. Adding a strong acid, like Hl, regenerates the carboxylic acid. Solubility Switch H 3 H NaH Hl H 3 _ Na + hapter 20: arboxylic Acids Slide

8 Naming Acid Salts Name the cation. Then name the anion by replacing the -ic acid with -ate. l H 3 H 2 HH 2 - K + potassium 3-chloropentanoate potassium β-chlorovalerate hapter 20: arboxylic Acids Slide Properties of Acid Salts Usually solids with no odor. arboxylate salts of Na +, K +, Li +, and NH 4 + are soluble in water. Soap is the soluble sodium salt of a long chain fatty acid. Salts can be formed by the reaction of an acid with NaH 3, releasing 2. hapter 20: arboxylic Acids Slide

9 Purifying an Acid hapter 20: arboxylic Acids Slide Some Important Acids Acetic acid is in vinegar and other foods, used industrially as solvent, catalyst, and reagent for synthesis. Fatty acids from fats and oils. Benzoic acid in drugs, preservatives. Adipic acid used to make nylon 66. Phthalic acid used to make polyesters. hapter 20: arboxylic Acids Slide

10 IR Spectroscopy hapter 20: arboxylic Acids Slide NMR Spectroscopy hapter 20: arboxylic Acids Slide

11 UV Spectroscopy Saturated carboxylic acids absorb very weakly around nm. If = is conjugated with =, molar absorptivity = 10,000 at 200 nm. An additional conjugated double bond increases the absorption wavelength to 250 nm. hapter 20: arboxylic Acids Slide Mass Spectrometry hapter 20: arboxylic Acids Slide

12 Synthesis Review xidation of primary alcohols and aldehydes with chromic acid. leavage of an alkene with hot KMn 4 produces a carboxylic acid if there is a hydrogen on the doublebonded carbon. leavage of an alkyne with ozone or hot permanganate. Alkyl benzene oxidized to benzoic acid by hot KMn 4 or hot chromic acid. hapter 20: arboxylic Acids Slide Grignard Synthesis Grignard reagent + 2 yields a carboxylate salt. H 3 H 3 H 3 HH 2 MgBr H 3 H 3 H 3 HH 2 - MgBr + H + H 3 H 3 H 3 HH 2 H hapter 20: arboxylic Acids Slide

13 Hydrolysis of Nitriles Basic or acidic hydrolysis of a nitrile produces a carboxylic acid. Br NaN N H + H 2 H hapter 20: arboxylic Acids Slide Acid Derivatives The group bonded to the acyl carbon determines the class of compound: -H, carboxylic acid -l, acid chloride -R, ester -NH 2, amide These interconvert via nucleophilic acyl substitution. hapter 20: arboxylic Acids Slide

14 Fischer Esterification Acid + alcohol yields ester + water. Acid catalyzed for weak nucleophile. All steps are reversible. Reaction reaches equilibrium. H + H 3 H 2 H H + H 2 H 3 + HH hapter 20: arboxylic Acids Slide Fischer Mechanism (1) Protonation of carbonyl and attack of alcohol, a weak nucleophile. H H + + H H H H + H 3 H 2 H H H + H H 2 H 3 H R H H H 2 H 3 hapter 20: arboxylic Acids Slide

15 Fischer Mechanism (2) Protonation of -H and loss of water. H + H H H + H H + H H R H 2 H 3 H 2 H 3 H 2 H 3 H 2 H 3 hapter 20: arboxylic Acids Slide Diazomethane H 2 N 2 reacts with carboxylic acids to produce methyl esters quantitatively. Very toxic, explosive. Dissolve in ether. H + H2 N 2 H 3 + N 2 hapter 20: arboxylic Acids Slide

16 Mechanism for Diazomethane hapter 20: arboxylic Acids Slide Amides from Acids Amine (base) removes a proton from the carboxylic acid to form a salt (simple acid/base reaction!). Heating the salt above 100 drives off steam and forms the amide. H + H 3 NH NH 3 H 3 heat NHH 3 + H 2 hapter 20: arboxylic Acids Slide

17 Reduction to 1 Alcohols Use strong reducing agent, LiAlH 4. Borane, BH 3 in THF, reduces carboxylic acid to alcohol, but does not reduce ketone. hapter 20: arboxylic Acids Slide Reduction to Aldehyde Difficult to stop reduction at aldehyde. Use a more reactive form of the acid (an acid chloride) and a weaker reducing agent, lithium aluminum tri(t-butoxy)hydride. l LiAl[(H 3 ) 3 ] 3 H H hapter 20: arboxylic Acids Slide

18 Alkylation to Form Ketones React 2 equivalents of an organolithium reagent with a carboxylic acid. H 1) 2 H 3 H 2 Li H 2 H 3 2) H 2 hapter 20: arboxylic Acids Slide Acid hlorides An activated form of the carboxylic acid. hloride is a good leaving group, so undergoes acyl substitution easily. To synthesize acid chlorides use thionyl chloride or oxalyl chloride with the acid. H + l l l Hl 2 hapter 20: arboxylic Acids Slide

19 Esters from Acid hlorides Acid chlorides react with alcohols to give esters in good yield. Mechanism is nucleophilic addition of the alcohol to the carbonyl as chloride ion leaves, then deprotonation. l + H 3 H H 3 + Hl hapter 20: arboxylic Acids Slide Amides from Acid hlorides Acid chlorides react with ammonia and amines to give amides. A base (NaH or pyridine) is added to remove Hl byproduct. l NHH 3 NaH + H 3 NH 2 + Nal + H 2 hapter 20: arboxylic Acids Slide

20 End of hapter 20 Homework: Practice naming anywhere you can! 33, 35, 36, 39, 42-44, 47, 48, 49 hapter 20: arboxylic Acids Slide