#halogenation

[er-cryptid]

Products of Free Radical Halogenation of Alkanes [Ex. 5]

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[molecular-tea]

Química Orgánica I: Halogenación

Se lleva a cabo por radicales libres. Un grupo X reemplaza a un hidrógeno del sustrato, el cual puede ser un alcano o porción de alcano:

Etapas:

Iniciación: Por acción de calor o luz (energía), se generan radicales libres a partir de una molécula de halógeno, como Cl2 o Br2.

Propagación: Uno de los radicales libres colisiona con un hidrógeno del alcano, entonces se forma un hidrácido y un radical alquilo.

Terminación: El radical alquilo reacciona con una molécula de halógeno para formar un halogenuro de alquilo y radical del halógeno. También puede suceder la colisión de dos radicales alquilo, formando así un nuevo alcano, pero esto sucede poco.

Estas etapas se ciclan hasta que dejen de producirse radicales libres en la última etapa.

↝En el caso de cloración, no hay una preferencia muy definida en cuanto a los tipos de hidrógenos, pero el producto ligeramente mayor que se produce es aquel donde se sustituye el H terciario. ↝En el caso de bromación, sí hay una preferencia muy marcada y es donde el producto mayoritario se da en H terciario.

Nota: Si la temperatura se aumenta mucho (cientos de grados centígrados), se producen todos los posibles productos de halogenación. Tomar en cuenta estereoisómeros, si es que pueden generarse.

[labphoto]

Some succinimide precitipated after a bromination of a toluene derivative using NBS (N-bromosuccinimide).

N-Bromosuccinimide or NBS is a chemical reagent used in radical substitution and electrophilic addition reactions in organic chemistry. NBS can be a convenient source of Br•, the bromine radical.

Interesting things from NBS: it is well soluble in chloroform and other halogenated solvents, but when the bromine reacts and succinimide is produced, it precipates as a white solid (what is seen at the side of the flask). Another info: NBS can be recrystallized from hot water.

[er-cryptid]

Mechanism of Free Radical Halogenation

Initiation -- first step -- generates free radicals -- done by heat or light -- these cleave σ bond -- cleaves a weak bond -- a halogen-halogen bond is weak

Propagation -- second step -- heart of the reaction -- major product is made

Termination -- last step -- destruction of radicals -- one way to stop the reaction

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[labphoto]

Chlorination with elemental chlorine and a low pressure mercury vapor lamp. 

This is the easiest and best way of introducing a halogen into a molecule, just a chlorine cylinder and some UV light is needed, the only thing that is produced as a byproduct is a HCl gas in this case:

It’s important to run photochlorinations in glassware that lets UV light in, since if the glass blocks the radiation that is needed to produce a large amount of chlorine radical from the chlorine molecules nothing will happen inside the flask. In this case I used Pyrex glassware that is perfect for most cases.

Important note: UV radiation besides it generates reactive species from chlorine molecules also damages eyes and skin, so in every case, these type of reactions should be covered with something (e.g. aluminium foil) to avoid any accident. 

[labphoto]

Bromination with acetic acid, second part.

A concentrated bromine solution in acetic acid (approximately 30%) is added in a slow rate to another solution of an aromatic compound in acetic acid in the flask. The bromine reacts slowly, but it’s clearly visible that it has a much faint color compared to the added concentrated solution. 

As the reaction happens, the color will completely fade and if I am enough lucky, the product will precipitate from the solution, so only a filtration is needed as an isolation step. A great part of bromination in acetic acid is, that the produced hydrogen bromide stays in solution, because acetic acid dissolves it quite well, so almost no toxic fumes are produced from the reaction. However care should be taken, since bromine is highly toxic/corrosive, just as the produced HBr solution in acetic acid.

The mechanism of the bromination in case of Lewis acid catalyst (FeBr3): 

[biology-geology-beaches-india]

The Scientific Research Notes Of S. Sunkavally (years: 2002-2011).

2999.

[er-cryptid]

EAS Halogenation [Ex. 1]

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