Los alcanos son los compuestos orgánicos más simples de los cuáles se derivan una serie compuestos más; y de ahí radica su gran importancia en el mundo de la química orgánica. Pero, ¿cómo se obtienen estos compuestos? A continuación te muestro cuatro métodos que se emplean en la preparación de alcanos.
1- Carburo de aluminio + Agua
Este procedimiento es bastante eficaz a la hora de obtener metano y consiste en la descomposición del carburo de aluminio (Al4C3) utilizando agua, como se muestra en la ecuación balanceada siguiente:
Al4C3 + 12 H2O → 3CH4 + 4 Al(OH)3
En la reacción se obtiene metano e hidróxido de aluminio.
2- Sal sódico de un ácido orgánico + Hidróxido de sodio
Cuando se utiliza este método, el alcano obtenido posee un átomo de carbono menos que la sal de la cual se origina. Entonces si deseas preparar un cierto hidrocarburo, se debe iniciar a partir de una sal con un átomo más de carbono. Por ejemplo: si se desea obtener el etano, se debe partir del propanoato de sodio, como te especifico a continuación:
CH3-CH2-COO-Na + NaOH → CH3-CH3 + Na2CO3
A partir de la combinación entre el propanoato de sodio y el hidróxido de sodio se produce el etano y carbonato de sodio.
3- hidrogenación de alquenos
Al adicionar hidrógeno a los alquenos en presencia de catalizadores como níquel, paladio o platino, se producen alcanos. Estos catalizadores son efectivos en condiciones de temperatura ordinarias y la presión a 1 atmósfera. Siendo el níquel es más efectivo de todos.
Este método consiste en combinar alqueno correspondiente, vaporizado con hidrógeno sobre níquel pulverizado a una temperatura bastante elevada. Dicho proceso es conocido como hidrogenación.
Por ejemplo, la hidrogenación del propeno en presencia de níquel como catalizador:
CH3-CH=CH2 + H2 → CH3-CH2-CH3
A partir del propeno obtendremos propano.
4- reducción de haluros de alquilo
I) reducción de haluros de alquilo por hidrólisis del reactivo de grignard.
El Reactivo de Grignard es un compuesto organometálico, constituido por la unión de un carbono a un metal. Esta sustancia resulta de la combinación entre una solución de un halogenuro de alquilo (R – X) con laminillas de magnesio (Mg) en solución de éter dietílico seco o anhidro [(C2H5)2O]. Posee como fórmula general RMgX, y su nombre general es halogenuro de alquilmagnesio.
Por hidrólisis del reactivo de Grignard, se obtiene el alcano correspondiente, según la ecuación general:
RMgX + H2O → R-H + MgX(OH)
Reactivo de Grignard →Alcano
Por ejemplo:
CH3-CH2-CH2-MgCl + H2O → CH3-CH2-CH3 + MgCl(OH)
Cloruro de n-propilmagnesio + agua → propano + Hidroxicloruro de magnesio
II) Reducción de haluros de alquilo por un ácido y un metal.
Los haluros o halogenuros de alquilo se pueden reducir para obtener alcanos, por la presencia de hidrógeno y un metal como zinc, además de la reacción entre un ácido como el ácido acético o el ácido clorhídrico; según la ecuación general:
Por ejemplo:
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