A search on quite a few of Lewis' works did not yield anything with the phrase "chemical reaction" in it. Additionally, searches for "love" and "chemical" revealed nothing as well.
However, Lewis is quoted this, which is along the same vein:
We can be certain that, in this life at any rate, thought is intimately connected to the brain. The theory that thought therefore is merely a movement of the brain is, in my opinion, nonsense, for if so, the theory itself would merely be a movement, an event among atoms, which may have speed and direction, but of which it would be meaningless to use the words "true" and "false."
It is reprinted in many places.1 2 3 It is very likely legitimately Lewis' words and was first said, not written, in a sermon in 1944, which was later transposed and published in a work titled Transposition and other addresses.4 According to one source, The Weight of Glory was the American version of that work.5 An actual copy of this sermon does not seem to exist online. But the consistency among quoters and the volume of those quoting leads to a reasonable conclusion that Lewis did indeed say these words.
The quote above seems to contain the essence of what you remember, which is that thoughts and emotions are more than chemical reactions, or 'movements in the brain'.
In general, there are many persons, past and present, that have posited and challenged the notion that "love is a chemical reaction". As you should expect, the idea was first forwarded by naturalists and has since been resoundingly rejected by many Christian theologians because of the implications on Christian theology.6
- C. S. Lewis's List: The Ten Books That Influenced Him Most
- A Philosophical Walking Tour with C.S. Lewis: Why it Did Not Include Rome
- A Mind Awake: An Anthology of C. S. Lewis
- C.S. Lewis: Apologist, philosopher, and theologian
- Lewis as Preacher – A 75th Anniversary Reflection - cslewis.com
- Love the chemical reaction - Encyclopedia of Human Thermodynamics, Human Chemistry, and Human Physics