Electrophilic Additions of Aldehydes

These are additions to double bonds, like the Prins reaction, and they can be carried out on natural and synthetic rubbers [59, 60]. They take place rapidly in the presence of acid catalysts. Aqueous formaldehyde [61], or paraform in CCl₄ [62], can be used. The catalysts are inorganic acids or anhydrous Lewis acids, like boron trifluoride in acetic acid solution [63]:

The reaction takes a different path in the absence of a catalyst [62]:

The products of the Prins reaction with rubbers are thermoplastic polymers that possess fair resistance to acids and bases. Free hydroxyl groups in the products are available for cross-linking with diisocyanates [64] or by other means. The Prins reaction can be carried out directly on rubber latexes [65]. It is also possible to just mill the rubber together with formaldehyde and then heat the resultant mixture in the presence of anhydrous metal chlorides [64] to get similar results [66]. Higher aldehydes also react with natural rubber [67]. The reaction works best with purified rubber. Additions take place without a catalyst at 180C or in the presence of AlCl3–NaCl at 120C. These reactions can be carried out in the solid phase by milling the rubber with an aldehyde, like glyoxal [68]. Heating in a pressure vessel at above 175C is required to complete the reaction. Infra-red spectra of the products from reactions in solution show presence of ether, carbonyl, and hydroxyl groups [69]. Two types of additions appear to take place [69]:

Products from reactions of rubber with glyoxal have a strong tendency to become spontaneously insoluble. This is probably due to a presence of residual aldehyde groups, because a treatment of the product with 2,4-dinitrophenyl-hydrazine eliminates spontaneous gelation. Chloral adds to polyisoprene similarly. The reaction is catalyzed by Lewis acids [70]. Both AlCl3 and BF3 are efficient catalysts. Less cross-linking is encountered with aluminum chloride. Infra-red spectra of the products shows presence of hydroxyl groups, chlorine atoms, and vinylidene unsaturation [70].

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الآخبار الصحية

مفاجأة.. تحسين القدرة على التحمل لا يعتمد العضلات فقط

المرتفعات والأكسجين.. دراسة "مبشرة" لمرضى السكري

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الآخبار التكنلوجية

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أسماك أعماق البحر تكشف نظاما بصريا يتجاوز "المألوف"

زوكربيرغ أمام القضاء في قضية إدمان المراهقين

"علي بابا" تكشف عن نموذج جديد للذكاء الاصطناعي

المزيد

مواضيع ذات صلة

Chloromethylation Reactions

Reactions of Polystyrene

Substitution Reactions of Halogen-Bearing Polymers

Substitution Reactions of Saturated Polymeric Hydrocarbons

Cyclizations and Intramolecular Rearrangements

Isomerization Reactions

Epoxidation Reactions

Diels–Alder Condensations

1,3-Dipolar Additions

Addition of Carbenes

Hydrogenation

Halogenation