Sandler / Karo Sourcebook of Advanced Organic Laboratory Preparations

1. Auflage 2012
ISBN: 978-0-08-092553-0
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 352 Seiten

In the case of students, this laboratory preparations manual can be used to find additional experiments to illustrate concepts in synthesis and to augment existing laboratory texts. A name reaction index is also included to direct the reader to the location where specific reactions appear in this manual. The industrial chemist is frequently required to prepare a variety of compounds, and this manual can serve as a convenient guide to choose a synthetic route. - Offers detailed directions for the synthesis of various functional groups - Includes up-to-date references to the journal literature and patents (foreign and domestic) - Reviews the chemistry for each functional group with suggestions where additional research is needed - Name reactions are indexed along with the preparations cited

Dr. Stanley R. Sandler won the R&D 100 Award offered by the industry in 1990 for a significant commercial process to prepare an important organic intermediate. In addition to this honor, he has over 100 publications involving patents, books, an encyclopedia article, several journal articles, and he is currently a referee for several journals. Sandler received his Ph.D. in Organic Chemistry from Penn State University.

Zielgruppe

Academic/professional/technical: Undergraduate. Academic/professional/technical: Postgraduate. Academic/professional/technical: Research and professional

Autoren/Hrsg.

Sandler, Stanley R.

Karo, Wolf

Leseproben

2

OLEFINS

Publisher Summary

This chapter discusses the reactions involved in the preparation of olefins. Olefins are prepared by elimination reactions and condensation reactions. Condensation reactions help to convert aliphatic aldehydes to substituted olefins via the preparation of dibromoethyl ethers, Grignard coupling, and elimination of bromoethoxy zinc. Aldehydes can be converted to olefins by reaction with active methylene compounds by Knoevenagel, Perkin, Claisen, and aldol condensation reactions. Wittig reaction is a convenient laboratory method useful for the conversion of aldehydes and ketones to olefins via the reaction of triphenylphosphinemethylenes. The condensation of acetylenes with carbon monoxide, hydrogen halides, alcohols, acids, amines, mercaptans, and halogens gives useful unsaturated compounds. Because of the hazards involved in handling acetylene, the laboratory workers avoid its use. However, Reppe’s pioneering experimental work has shown that under proper conditions, the hazards associated with acetylene may be minimized and that acetylene is quite useful for vinylation reactions. Some industrial processes utilize acetylene reactions to prepare vinyl ethers, acrylic acid, vinyl fluoride, 2-butene-l, 4-diol, and some other olefins used for the preparation of plastics.

1 INTRODUCTION

2 ELIMINATION REACTIONS

A. Dehydration of Alcohols

2-1 Preparation of Cyclohexene

B. Dehydrohalogenation Reactions

2-2 Preparation of 3-Chloro-2-methyl- and 3-Chloro-4-methyl-a-methylstyrene

C. The Wittig Synthesis of Olefins

2-3 Preparation of 1,2-Distyrylbenzene

D. Condensation of Aldehydes and Ketones with Themselves or with Other Active Methylene Compounds

2-4 Aldol Condensation—Preparation of a-Ethylcinnamaldehyde

1 INTRODUCTION

Olefins are commonly prepared by elimination reactions (loss of water, hydrogen halides, acids, etc.) and condensation reactions. Methods utilizing oxidation, reduction, isomerization or rearrangement, free radical, photolytic, and enzyme reactions are less commonly used in the laboratory to prepare a center of unsaturation.

The elimination reactions are summarized by Eq. (1)

(1)

where Z may be a hydroxyl, halogen, ester, ether, methyl xanthate (Chugaev reaction), carbamate, carbonate, sulfite, amine, quaternary ammonium hydroxide (Hofmann degradation), amine oxide, or one of many other labile groups.

Another elimination reaction involves disubstituted derivatives as in Eq. (2)

(2)

where Z may be a hydroxyl or halogen.

Condensation reactions, such as the Boord synthesis, are good methods for converting aliphatic aldehydes [Eq. (3)] to substituted olefins via the preparation of dibromoethyl ethers, Grignard coupling, and elimination of bromoethoxy zinc. (See the referenced text for more detailed equations.)

(3)

Aldehydes can also be converted to olefins by reaction with active methylene compounds [Eq. (4)] by the Knoevenagel, Perkin, Claisen, and aldol condensation reactions

(4)

where R’s are carboxylic acid or ester groups, nitro, nitrile, carboxylic anhydride groups, aldehydes, and ketones or any other strongly electron-withdrawing substituents.

The Wittig reaction [Eq. (5)] is a convenient laboratory method useful for the conversion of aldehydes and ketones to olefins via the reaction of triphenylphosphinemethylenes

(5)

where X is hydrogen, alkyl, alkyl carboxylate, and halogen.

A convenient modification of the Wittig method uses phosphonate carbanions, (RO)2P- OCX2, in place of (C6H5)3 P=CX2.

The condensation of acetylenes with carbon monoxide, hydrogen halides, alcohols, acids, amines, mercaptans, halogens, etc., gives extremely useful unsaturated compounds as generalized by Eq. (6).

(6)

However, as a result of the hazards involved in handling acetylene, a great many laboratory workers avoid its use. Nevertheless, Reppe’s pioneering experimental work has shown that under the proper conditions the hazards associated with acetylene may be minimized and that acetylene is quite useful for vinylation reactions. Some industrial processes today utilize acetylene reactions to prepare vinyl ethers, acrylic acid, vinyl fluoride, 2-butene-1,4-diol, and some other olefins used for the preparation of plastics.

2 ELIMINATION REACTIONS

A DEHYDRATION OF ALCOHOLS

The acid-catalyzed or thermal elimination of water from alcohols is a favorite laboratory method for the preparation of olefins. Isomeric mixtures usually arise with the acid-catalyzed method. The order of reactivity in dehydration usually follows the order of stability of the intermediate (transient) carbonium ion, i.e., tertiary > secondary > primary. The acid-catalyzed procedure is illustrated below, where a 79–87% yield of cyclohexene is obtained [1–3].

Tertiary arylcarbinols have been reported to be converted within 30 sec to the corresponding alkenes (70% yield) with warm 20% sulfuric–acetic acid (by volume) [4]. The yields are much lower with aliphatic tertiary or secondary arylcarbinols. Some tertiary alcohols, such as those obtained from tetralone and the Grignard reagent, dehydrate on simple distillation and in the presence of anhydrous cupric sulfate as a catalyst [5].

Secondary and tertiary alcohols can be dehydrated in dimethyl sulfoxide when heated to 160°-185°C for 14–16 hr to give olefins in yields of 70–85% [6]. The solution is diluted with water, extracted with petroleum ether (30°-60°C), dried, and then distilled. Other acid catalysts that have been reported for dehydration of alcohols are: anhydrous or aqueous oxalic [7, 8], or phosphoric acid [9], and potassium acid sulfate [10, 11]. In addition, acidic oxides such as phosphorus pentoxide [10–12] and acidic chlorides such as phosphorus oxychloride or thionyl chloride [13] have been reported to be effective as catalysts for the dehydration reaction.

2–1 Preparation of Cyclohexene [3]

(7)

To 400 gm (4 moles) of cyclohexanol, in a flask set up for distillation of the contents into an ice-cooled receiver, is added 12 ml of concentrated sulfuric acid. The flask is heated to 130°-150°C by means of an oil bath for 5–6 hr. Water and crude cyclohexene are distilled out of the reaction mixture. The cyclohexene is salted out of the distillate, dried, and fractionated to give 260–285 gm (78–87%), bp 80°-82°C.

B DEHYDROHALOGENATION REACTIONS

The dehydrohalogenation reaction is complex because the nucleophile B, can remove the ß-proton to produce elimination, it can attack the a-carbon to give the 2 product or give a-elimination. Normally a-elimination leads to the same product as obtained by ß-elimination.

With simple unbranched alkyl halides, the use of alcoholic bases gives the Saytzeff olefin. Steric effects on the ß-position usually increases Hofmann elimination.

(8)

(9)

2–2 Preparation of 3-Chloro-2-methyl- and 3-Chloro-4-methyl-a-methylstyrene [14]

(10)

Four hundred and eight grams (2.0 moles) of chloropropylated -chlorotoluene (from propylene chlorhydrin, -chlorotoluene, boron trifluoride, and phosphorus pentoxide) is refluxed with an 85% solution of potassium hydroxide in methanol [392 gm (7 moles) of KOH in 1850 ml of methanol]. The methanol is removed by distillation and the remaining liquid is washed with water, dried with calcium chloride, and distilled through an efficient column under reduced pressure to give 90 gm (26%) of 3-chloro-2-methyl-a-methylstyrene, bp 64°-65°C (4 mm), 1.5340 and 152 gm (48%) of 3-chloro-4-methyl-a-methylstyrene, bp 73°-74°C (4 mm), 1.5520. The purity of these materials should be checked by gas chromatography.

C THE WITTIG SYNTHESIS OF OLEFINS [15–20]

In 1953 Wittig and Geissler discovered that methylenetriphenyl-phosphorane reacted with benzophenone to give 1,1-diphenylethylene and triphenylphosphine oxide in almost quantitative yield. The phosphorane was prepared from triphenylmethylphosphonium bromide and phenyllithium.

(11)

The advantage of the Wittig method...