Anidrido

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Anidridos são compostos resultantes da desidratação de ácidos.

Classificação

Existem duas grandes classes: Inorgânicos ou Minerais e Orgânicos. Vejamos suas diferenças.

Inorgânicos ou Minerais

Exemplo

H_{2}SO_{4}\rightarrow H_{2}O+SO_{3}\uparrow

Ácido sulfúrico $\rightarrow$ água + anidrido sulfúrico

Para se obter o anidrido sulfúrico basta aquecer o ácido até seu ponto de ebulição e recolher seus vapores.

Alguns ácidos, como o ácido carbônico, se decompõem em temperatura ambiente. O gás carbônico também é conhecido como anidrido carbônico.

H_{2}CO_{3}\rightarrow H_{2}O+CO_{2}\uparrow

Orgânicos

Ver artigo principal: Anidrido de ácido carboxílico

São compostos formados por dois grupos acil ligados ao mesmo átomo de oxigênio.^[1] Um caso particular são os anidridos sulfônicos, de estrutura RS(=0)₂OS(=0)₂R'.^[2] Possuem cadeia carbônica e que podem se subdividir em:

Anidridos comuns

São aqueles em que as cadeias carbônicas são iguais nos anidridos (C=C).

Exemplo

\ CH_{3}-CO-O-CO-CH_{3}

Anidridos Mistos

São aqueles cuja cadeia carbônica tem diferente número de átomos de carbono ou seja (C $\neq$ C).

Exemplo

\ CH_{3}-CO-O-CO-CH_{2}-CH_{3}

Podemos ver a seguir como se forma um anidrido orgânico:

Anidridos acíclicos (de monoácidos). Exemplo:

e Anidridos cíclicos (de diácidos).Exemplo:

Propriedades

Físicas

As moléculas dos anidridos são polares mas não fazem ligações de hidrogênio, assim sendo possuem ponto de fusão e ponto de ebulição mais baixos que os dos álcoois e dos ácidos carboxílicos correspondentes. São mais densos que a água e pouco solúveis nela, sendo no entanto mais solúveis em solventes orgânicos. Os anidridos de baixo peso molecular são líquidos de cheiro irritante e forte.

Químicas

Os anidridos acíclicos reagem com álcoois formando ésteres e o ácido correspondente e com amoníaco formando amida e sal orgânico de amônio. Veja as reações:

Tipos de Anidrido

Anidrido com álcool

\ (CH_{3}-CO)_{2}O+C_{2}H_{5}OH\rightarrow CH_{3}-COO-C_{2}H_{5}+CH_{3}-COOH

Anidrido acético + álcool acetato de etila + ácido acético

Anidrido com amoníaco

\ (CH_{3}-CO)_{2}O+2NH_{3}\rightarrow CH_{3}-CO-NH_{2}+CH_{3}-COONH_{4}

Anidrido acético + amoníaco $\rightarrow$ acetamida + acetato de amônio

Reação de Friedel-Crafts

Os compostos de anéis aromáticos (no caso, benzeno) reagem com o anidrido acético na famosa reação de Friedel-Crafts, originando cetonas, sendo que metade da molécula forma a acila e a outra metade resulta no ácido carboxílico.

\ (CH_{3}-CO)_{2}O+C_{6}H_{6}\rightarrow C_{6}H_{5}-CO-CH_{3}+CH_{3}-COOH

Anidrido acético + benzeno $\rightarrow$ acetofenona + ácido acético

Condensação de Perkin

Os aldeídos aromáticos, em presença de hidróxidos, reagem com anidridos formando ácidos insaturados $\alpha ,\beta$ .

Reações de anidridos cíclicos

Reagem com álcoois formando éster-ácido.

Exemplos

Os anidridos Succínico e o-Ftálico reagem da mesma forma, no entanto, por terem uma cadeia cíclica carbono-carbono o composto acilo e ácido carboxílico estarão na mesma molécula, contendo ao mesmo tempo estes grupos funcionais.

Anidrido Succínico

Anidrido Ftálico

Ver também

Referências

↑ IUPAC, Compêndio de Terminologia Química, 2ª ed. ("Gold Book"). Compilado por A. D. McNaught e A. Wilkinson. Blackwell Scientific Publications, Oxford (1997). Versão online: "acid anhydrides" (2006–) criado por M. Nic, J. Jirat, B. Kosata; atualizações compiladas por A. Jenkins. ISBN 0-9678550-9-8.
↑ IUPAC, Compêndio de Terminologia Química, 2ª ed. ("Gold Book"). Compilado por A. D. McNaught e A. Wilkinson. Blackwell Scientific Publications, Oxford (1997). Versão online: "sulfonic anhydrides" (2006–) criado por M. Nic, J. Jirat, B. Kosata; atualizações compiladas por A. Jenkins. ISBN 0-9678550-9-8.

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