Method of reducing benzaldehyde or benzotrihalide content in a mixture

Disclosed is a method of removing about 0.05 to about 20 mole % of a benzaldehyde or benzotrihalide from a mixture with a liquid compound that boils within about -10 to about .+-.5.degree. C. of the boiling point of the benzaldehyde or benzotrihalide. To the mixture is added a benzotrihalide in an amount about stoichiometric to about 10 mole % in excess of stoichiometric, or, respectively, a benzaldehyde in an amount about stoichiometric with the amount of said benzotrihalide, where said benzaldehyde and said benzotrichloride react to form a benzalhalide and a benzoylhalide. The product mixture is distilled to isolate the products.

BACKGROUND OF THE INVENTION
 This invention relates to a method of reducing the benzaldehyde or
 benzotrihalide content in a mixture with a compound that boils at about
 the same temperature. In particular, it relates to adding a benzotrihalide
 or a benzaldehyde to the mixture to react with the benzaldehyde or a
 benzotrihalide, respectively, in order to form products having a greater
 difference in boiling point and therefore are more easily separated by
 distillation.
 When 2,4-dichlorotoluene (2,4-DCT) is reacted with chlorine in the presence
 of ultraviolet light a mixture of 2,4-dichlorobenzylchloride (2,4-DCBC)
 and 2,4-dichlorobenzalchloride (2,4-DCBAC) can be made. If the reaction
 mixture is exposed to moisture, however, some of the 2,4-DCBAC is
 hydrolyzed to 2,4-dichlorobenzaldehyde (2,4-DCBAL). When the product
 mixture is distilled, the 2,4-DCBC contains 2,4-DCBAL due to a relative
 volatility of close to one (boiling points) =248 and 250.degree. C.,
 respectively). The presence of 2,4-DCBAL in the 2,4-DCBC is not acceptable
 for uses such as making stabilizers, but separation of the 2,4-DCBAL by
 distillation is time-consuming and costly.
 When p-chlorotoluene (PCT) is chlorinated, a mixture of
 p-chlorobenzalchloride (PCBAC) and p-chlorobenzotrichloride (PCBTC) can be
 made. Separating the PCBAC by distillation is difficult due to the low
 relative volatility between PCBAC and PCBTC, reduces distillation yield.
 SUMMARY OF THE INVENTION
 We have found that a benzaldehyde can be separated from a mixture with a
 desired compound that boils at about the same temperature by reacting the
 benzaldehyde with a benzotrihalide. That reaction forms a benzalhalide and
 a benzoylhalide, which have substantially different boiling points than
 the desired compound, so the desired compound can be more easily separated
 by distillation.
 Similarly, a benzotrihalide can be more easily separated from a compound
 that boils at about the same temperature by reacting the benzotrihalide
 with a benzaldehyde to form a benzalchloride and a benzoylchloride, which
 have significantly different boiling points.
 DESCRIPTION OF THE PREFERRED EMBODIMENTS
 This invention is applicable to benzotrihalides and benzaldehydes having
 the respective general formulas:
 ##STR1##
 where each A is independently selected from halogen, CF.sub.3, R, or OR,
 each B is independently selected from halogen, CF.sub.3, or R, R is alkyl
 from C.sub.1 to C.sub.10 or aryl from C.sub.6 to C.sub.12, m is 0 to 3,
 and n is 0 to 2. Preferably, A is Cl or CF.sub.3, B is Cl, R is alkyl from
 C.sub.1 to C.sub.6, m is 0 to 2, and n is 0 or 1. Examples of
 benzotrihalides that can be used include benzotrichloride (BTC),
 o-chlorobenzotrichloride (OCBTC), ---chlorobenzotrichloride (MCBTC),
 p-chlorobenzotrichloride (PCBTC), 2,4-dichlorobenzotrichloride
 (2,4-DCBTC), and 3,4-dichlorobenzotrichloride (3,4-DCBTC),
 2,5-dichlorobenzotrichloride (2,5-DC BTC). The preferred benzotrihalides
 are 2,4-DCBTC, 3,4-DCBTC, OCBTC, MCBTC, and PCBTC. Examples of
 benzaldehydes that can be used include benzaldehyde (BAL),
 o-chlorobenzaldehyde (OCBAL), m-chlorobenzaldehyde (MCBAL), and
 p-chlorobenzaldehyde (PCBAL). The preferred benzaldehydes are BAL and
 OCBAL.
 The compound to be removed, either the benzaldehyde or the benzotrihalide,
 constitutes about 0.05 to about 20 mole %, and more typically about 1 to
 about 5 mole %, of a mixture with a compound that has a boiling point
 within -10 to +5.degree. C. of the boiling point of the benzotrihalide or
 benzaldehyde, and preferably within about -5 to about +5.degree. C. of
 that boiling point. Typically, the other compound in the mixture will be
 the corresponding benzotrihalide, benzaldehyde, benzylhalide,
 benzalhalide, or benzoylhalide.
 As an example, 2,4-DCBAL can be separated from a mixture of 2,4-DCBC and
 2,4-DCBAC by adding 2,4-DCBTC, which reacts with the 2,4-DCBAL to form
 additional 2,4-dichlorobenzalchloride (2,4-DCBAC) and
 2,4-dichlobenzoylchloride (2,4-DCBOC). Since the 2,4-DCBAC and the
 2,4-DCBOC are higher boiling than the 2,4-DCBC, the 2,4-DCBC can now be
 more easily isolated by distillation.
 As another example, PCBAC can be more easily separated from a mixture with
 PCBTC by reacting the PCBTC with PCBAL to form additional PCBAC and
 p-chlorobenzoylchloride (PCBOC). The PCBOC is higher boiling than the
 PCBAC.
 To remove a benzaldehyde from a mixture, the amount of benzotrichloride
 added should be about stoichiometric with the amount of benzaldehyde
 present in the mixture up to about 10 mole % in excess of stoichiometric;
 preferably, the amount of benzotrichloride added is about 1 to about 5
 mole % in excess of stoichiometric. If a benzaldehyde is added to a
 mixture to remove a benzotrihalide, the amount of benzaldehyde used should
 be at least stoichiometric with the amount of benzotrichloride in the
 mixture. To simplify the separation of products, it is preferable to use
 the corresponding benzaldehyde and benzotrihalide. The benzotrihalide or
 benzaldehyde can be added before, during, or after a reaction that forms
 the benzaldehyde or benzotrihalide, respectively.
 The reaction between the benzaldehyde and the benzotrihalide occurs at room
 temperature; higher temperatures due to the accumulation of reaction heat
 or to external heating will accelerate the reaction. No catalyst is
 needed, though about 0.01 to about 1 wt % of a Lewis acid catalyst, such
 as zinc chloride or ferric chloride, can be used to accelerate the
 reaction if desired. Before distilling, the reaction should be held at an
 elevated temperature for a period of time to permit the reaction between
 benzaldehyde and benzotrichloride to occur. The reaction time, reaction
 temperature, and catalyst level directly influence the reaction rate and,
 therefore, the level of benzaldehyde or benzotrichloride in the mixture.
 The following examples further illustrate this invention:

EXAMPLE 1
 To prepare a test mixture, 2.32 g (0.27 wt %) of 2,4-DCBAL was added to 860
 g of a 85:15 wt % mixture of 2,4-DCBC:2,4-DCBAC. Distillation in a
 30-plate.times.1" ID Oldershaw column at reflux ratios ranging from 10:1
 to 25:1 over 11.25 hours resulted in a loss of 148.4 g of the desired
 2,4-DCBC product in the foreshot due to 2,4-DCBAL contamination. The final
 distillate contained 0.51% (GC area %) 2,4-DCBAL. When the distillation
 was repeated, but with 26.6 g 2,4-DCBTC (11.5 wt excess or 7.6 molar
 excess based on 2,4-DCBAL), the 2,4-DCBAL level in the distillate was
 reduced to a non-detectable level. Only 26.5 g of contaminated 2,4-DCBC
 was collected in the foreshot over 36 hours, of which 12.75 hrs was the
 reaction time at 160.degree. C. before the distillation was started.
 EXAMPLE 2
 To prepare a test mixture, 2.44 9 (0.28 wt %) of 2,4-DCBAL was added to 860
 g of a 85:15 wt % mixture of 2,4-DCBC:2,4-DCBAC. In addition, 11.52 g of
 2,4-DCBTC (3.12 molar excess based on 2,4-DCBAL) and 0.8589 g (1000 ppm)
 of zinc acetate catalyst were added to the reboiler. To induce reaction in
 the reboiler, the mixture was held at 160.degree. C. for approximately 3
 hours. The 2,4-DCBAL level was reduced and only 66 g of 2,4-DCBC was
 contaminated with 2,4-DCBAL, which was collected in the foreshot over 7.65
 hours at 20:1 reflux ratio; 2,4-DCBAL was not detectable in the final
 distillate.
 EXAMPLE 3
 A 1283 g mixture containing 0.38 gmoles PCBC, 4.45 gmoles PCBAC and 1.53
 gmoles PCBTC was reacted with 1.774 BAL in the presence of zinc acetate
 catalyst at 160.degree. C. The reaction product analyses showed 1.77
 gmoles benzalchloride (BAC), 1.214 gmoles PCBOC, 4.37 gmoles PCBAC and
 0.19 gmoles PCBAL. Upon distillation in a 30-plate.times.1" Oldershaw
 column, 99.81% pure PCBAC was recovered, resulting in 81% yield.