Acid chlorides resonance forms

In Friedel-Crafts acylation, the Lewis acid AICI3 ionizes the carbon-halogen bond of the acid chloride, thus forming a positively charged carbon electrophile called an acylium ion, which is resonance stabilized (Mechanism 18.7). The po.sitively charged carbon atom of the acylium ion then goes on to react with benzene in the two-step mechanism of electrophilic aromatic substitution. 

Therefore, the polar resonance form contributes most in amides, and they are the most stabilized. Second, in acid chlorides, resonance stabilization requires overlap between a 7>p orbital on chlorine and the carbonyl Jt orbital made up of 2p orbitals. In the other acid derivatives the overlap is between 2p orbitals. 

Medium Acidic. Sources The lone pairs on the carbonyl are the best source (much better than the lone pairs of the OH). Leaving groups Chloride. Sinks The best, SOCI2, is a Y-L. The carboxylic acid is both an acid and a carboxyl derivative sink, but the OH is a poor leaving group. Acidic Hs The carboxylic acid s OH. Bases None. Resonance forms By VSEPR SOCI2 is tetrahedral, often drawn with an expanded octet resonance form containing a d-p pi bond. 

The resistance of cyclopropanone to ring opening by hydrogen chloride and acetic acid may be attributed to the strong contribution of resonance form 3 which localizes the electron deficiency on oxygen rather than on carbon. 

In principle, this reaction occurs with carboxylic acids, where X = OH in 72. The reaction occurs for carboxylic acid derivatives 56-60, where the X group in 72 and in 73 is Cl for an acid chloride, O2CR for an anhydride, OR for an ester, or NR2 for an amide. For these derivatives 73, a heteroatom is attached to the 6+ carbon, and there is additional resonance stabilization due to the lone electron pairs on these atoms. How oxocarbenium ions might react after they are formed has not been discussed, but it is clear that the carbonyl group of an acid derivative can react as a base in the presence of an appropriate acid. Reactions of oxocarbenium ions are discussed in Chapters 18 and 20. 

Acid chlorides react with benzene under the influence of an equivalent (not a catalytic amount) of AICI3. A molecule of AICI3 is needed for each molecule of product. By now it should be easy to write a general mechanism. A complex is first formed with AICI3. But which complex Unlike alkyl chlorides, acid chlorides contain two nucleophilic sites, the oxygen and chlorine atoms. The evidence is that both complexes are formed, and that the two are in equilibrium. The resonance-stabilized acylium ion can be formed by dissociation of the complex (Fig. 14.43). 

First of all, the positive charge in the polar resonance form is better accommodated by the relatively electropositive nitrogen of amides than by the more electronegative oxygen of esters and acids, or the chlorine of acid chlorides. 

So, amides are the most stabilized, acid chlorides the least stabilized, and the other acid derivatives come in between. Figure 18.14 shows the resonance forms for these compounds as well as for the familiar aldehydes and ketones. 

The acid chloride carbonyl is the strongest of the acid derivatives, with the most double-bond character, so its C O stretch appears at the highest frequency. Anhydrides, esters, and carboxylic acids are next. Amides, in which the contribution from the dipolar resonance form is strongest, and the sing/e-bond character of the carbonyl group is greatest, have the lowest carbonyl stretching frequencies (Table 18.2). 

As mentioned before, all acyl compounds participate in the addition-elimination process. Acid chlorides are especially reactive toward nucleophiles. Their carbonyl groups, being the least stabilized by resonance, have the highest energy and are the most reactive. So, an initial addition reaction with a nucleophile is relatively easy. The chloride atom of acid chlorides is an excellent leaving group, and sits poised, ready to depart once the tetrahedral intermediate has been formed... 

Chlorine does not effectively donate electrons by resonance via its 3p orbitals. Thus, the dipolar resonance form of a carbonyl group is less important, and the carbonyl group has more double bond character than an aldehyde or ketone. The chlorine atom also inductively withdraws electrons, destabilizing the dipolar resonance form. As a result, the carbonyl infrared absorption for an acid chloride requires more energy than for an aldehyde or ketone, and occurs at a higher wavenumber position. 

In the first step, the acid chloride reacts with AICI3 to form a resonance-stabilized acyhum ion (4). In the absence of an aromatic ring, the C=C n bond wiU function as a nucleophile and trap the acyhum ion to produce a carbocation (5). This carbocation is transformed into compoimd 1 if AICI4 transfers a chloride ion to the carbocation (path A). Alternatively, carbocation 5 is transformed into compound 2 via a 1,2-hydride shift to form tertiary carbocation 6 (path B), followed by deprotonation. 

Phenol condenses with phthahc anhydride in the presence of concentrated sulphuric acid or anhydrous zinc chloride to yield the colourless phenolphthalein as the main product. When dilute caustic alkah is added to an alcoholic solution of phenolphthalein, an intense red colouration is produced. The alkali opens the lactone ring in phenolphthalein and forms a salt at one phenolic group. The reaction may be represented in steps, with the formation of a h3q)othetical unstable Intermediate that changes to a coloured ion. The colour is probably due to resonance which places the negative charge on either of the two equivalent oxygen atoms. With excess of concentrated caustic alkali, the first red colour disappears this is due to the production of the carbinol and attendant salt formation, rendering resonance impossible. The various reactions may be represented as follows ... 

The reaction is less selective than the related benzoylation reaction (/pMe = 30.2, cf. 626), thereby indicating a greater charge on the electrophile this is in complete agreement with the greater ease of nuclophilic substitution of sulphonic acids and derivatives compared to carboxylic acids and derivatives and may be rationalized from a consideration of resonance structures. The effect of substituents on the reactivity of the sulphonyl chloride follows from the effect of stabilizing the aryl-sulphonium ion formed in the ionisation step (81) or from the effect on the preequilibrium step (79). 

Unlike cyclohexene, its oxa analog, 3,4-dihydro-2//-pyran, undergoes facile reduction to tetrahydropyran in yields ranging from 70 to 92% when treated with a slight excess of triethylsilane and an excess of either trifluoroacetic acid or a combination of hydrogen chloride and aluminum chloride (Eq. 69).146 This difference in behavior can be understood in terms of the accessibility of the resonance-stabilized oxonium ion intermediate formed upon protonation. 

Thermal decomposition of allylbenzene ozonide (58) at 37°C in the liquid phase gave toluene, bibenzyl, phenylacetaldehyde, formic acid, (benzyloxymethyl)formate, and benzyl formate as products. In chlorinated solvents, benzyl chloride is also formed and in the presence of a radical quench such as 1-butanethiol, the product distribution changes. Electron spin resonance (ESR) signals are observed in the presence of spin traps, adding to the evidence that suggests radicals are involved in the decomposition mechanism (Scheme 9) <89JA5839>. 

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