Eloquent get only one column as an array

You can use the pluck method:

Word_relation::where('word_one', $word_id)->pluck('word_two')->toArray();

For more info on what methods are available for using with collection, you can you can check out the Laravel Documentation.

If you receive multiple entries the correct method is called lists.

    Word_relation::select('word_two')->where('word_one', $word_id)->lists('word_one')->toArray();