Group 13 Lewis acids

Key points: The ability of boron trihalides to act as Lewis acids generally increases in the order BF3 < BCl3 < BBr₃; aluminium halides are dimeric in the gas phase and are used as catalysts in solution. The planar molecules BX3 and AlX3 have incomplete octets, and the vacant p orbital per pendicular to the plane (17) can accept a lone pair from a Lewis base:

The acid molecule becomes pyramidal as the complex is formed and the B–X bonds bend away from their new neighbours. The order of thermodynamic stability of complexes of :N(CH₃)₃ with BX3 is BF₃ < BCl₃ < BBr₃. This order is opposite to that expected on the basis of the relative electronegativities of the halogens: an electronegativity argument would suggest that F, the most electron egative halogen, ought to leave the B atom in BF3 most electron deficient and hence able to form the strongest bond to the incoming base. The currently accepted explanation is that the halogen atoms in the BX₃ molecule can form π bonds with the empty B2p orbital (18), and that these π bonds must be disrupted to make the acceptor orbital available for complex formation. The π bond also favours the planar structure of the molecule, a structure that must be converted into tetrahedral in the adduct. The small F atom forms the strongest π bonds with the B2p orbital: recall that p–p π bonding is strongest for Period 2 elements, largely on account of the small atomic radii of these elements and the significant overlap of their compact 2p orbitals (Section 2.5). Thus, the BF3 molecule has the strongest π bond to be broken when the amine forms an N–B bond. Boron trifluoride is widely used as an industrial catalyst. Its role is to extract bases bound to carbon and hence to generate carbocations:

Boron trifluoride is a gas at room temperature and pressure, but it dissolves in diethyl ether to give a solution that is convenient to use. This dissolution is also an aspect of Lewis acid character because, as BF₃ dissolves, it forms a complex with the :O atom of a solvent molecule. Aluminium halides are dimers in the gas phase; aluminium chloride, for example, has molecular formula Al₂Cl₆ in the vapour state (19). Each Al atom acts as an acid towards a Cl atom initially belonging to the other Al atom. Aluminium chloride is widely used as a Lewis acid catalyst for organic reactions. The classic examples are Friedel–Crafts alkylation (the attachment of R to an aromatic ring) and acylation (the attachment of RCO) during which AlCl^4– is formed. The catalytic cycle is shown in Fig. 4.9.

اخر الاخبار

اخبار العتبة العباسية المقدسة

تزامنًا مع حلول شهر ربيع الأول.. العتبة العباسية المقدسة ترفع مظاهر الحزن والسواد في المرقد الطاهر

بعد انتهاء شهري المحرم وصفر.. قسم الصيانة يباشر إدامة الثريات واستبدال مصابيح المرقد الطاهر

قسم الشؤون الخدمية يشرع بإدامة آليّات النظافة عقب انتهاء زيارة الأربعين

موكب عزاء الشعراء والرواديد ينظم مسيرة عزائية في ختام شهري الأحزان المحرم الحرام وصفر

المزيد

الآخبار الصحية

العنب.. فاكهة صغيرة بمفعول "خارق" للقلب والدماغ

دراسة تحدد "الحلقة المفقودة".. تسبب ارتفاع ضغط الدم

هذه الأطعمة تحمي دماغك من الشيخوخة وتزيد حدة الذكاء

خبراء يحذرون من مخاطر "الدهون المخفية" على القلب

المزيد

الآخبار التكنلوجية

الأرصاد الجوية تلجأ إلى الذكاء الاصطناعي.. لأي غرض؟

في منزلك.. "تعديل بسيط" يخفض ضغط الدم بشكل طبيعي

"ميزة جديدة" من إنستغرام تنافس تيك توك.. طال انتظارها

غوغل في مأزق.. بيانات 2.5 مليار مستخدم بقبضة القراصنة

المزيد

مواضيع ذات صلة

The classification of acids and bases

Halogens as Lewis acids

Group 14 Lewis acids

Lewis acids and bases of the s-block elements

Lewis acidity

Anhydrous sulfuric acid

Hydrogen fluoride

Liquid ammonia

Polyoxo compound formation

Amphoterism

Acidic and basic oxides

Periodic trends in aqua acid strength