def unique_reactions(df):
    """Identifies unique elementary reactions in data frame.

    Parameters
    ----------
    df : Data frame.

    Returns
    -------
    reaction_list : List of unique elementary reactions.
    """
    reaction_list =[]
    for idx, entry in enumerate(df['reactants']):
        reaction = []
        for x in entry:
            reaction.append(x)
        reaction.append('-->')
        for y in df['products'][idx]:
            reaction.append(y)
        reaction_list.append(reaction)
    string_list = [str(reaction) for reaction in reaction_list]
    string_list = sorted(list(set(string_list)))
    reaction_list = [ast.literal_eval(entry) for entry in string_list]
    return(reaction_list)