When is “al” not interchangeable with “a el”?

Question

This is a clear case where "al" cannot be replaced with "a el":

Al mirarlo, sonrió.

Are there any other cases?

Answer 1

To put this answer into simplest terms: No, al can't always replace a el.

Before an infinitive of a verb, al means upon or when. The pattern for this usage would be as follows:

Al <infinitive of X> <result Y> => Upon X, Y.

Examples:

• Al llegar a mi casa, voy a dormir. => When I get home I'm going to sleep.
• Al alcanzar su meta, se retiró. => Upon reaching his goal, he retired.

Notes:

• In this sense al doesn't mean when in a habitual sense (e.g. whenever.) In those cases cuando would be preferred.
• The al clause doesn't necessarily need to be placed first in the sentence, and for stylistic purposes, it can be moved around. That said, in plain speech it will almost always be the first element in the sentence.

In this usage it can not be interchanged arbitarily with a el. Oddly enough though, very few dictionaries make mention of this.

I hope this helps! :)

References:

• "Google Answers: ... Al before Infinative Verb." Web. 23 Apr. 2012. <http://answers.google.com/answers/threadview?id=483418>.

Answer 2

Some proper nouns that start with el don't contract. Back in the day, I worked near a taquería called El Pelón, and we'd say Vamos a El Pelón. But if you're in Chilangolandia and are taking a day trip to the mountains south of the city, you'd say Vamos al Ajusco.

The pattern for proper nouns, therefore, seems to be whether el is capitalized or not. (I'm not entirely sure about this, and would appreciate confirmation or refutation.)