So I want to put alder chippings in with chickens and rabbits. I know cedar and pine emit phenols and are bad, but I don't know anything about alders... or really what phenols are either. I guess they are the aromatic scent.
Do alders emit phenols and would they be toxic to animals? It would be the wood chippings. They wouldn't be burnt or anything.
Phenol is benzene with an extra oxygen tucked in to make it hydroxy-benzene. Phenol is a basic building block of many, many organic molecules and is not inherently bad, it depends on what else is in the molecule and what it might react with. If it has an extra carboxylic acid group in the molecule, you have salicylic acid, the compound in willow bark that aspirin is made from.
Phenol is not generally found in nature since it is quite reactive; it was first extracted by distillation from coal tar, and can be made commercially by a variety of routes from fossil fuels. It is used in all manner of chemical processes, from plastics to pharmaceuticals to antiseptics. Good ol' Campho-Phenique is camphor and phenol in a mineral oil carrier and has been used on cuts, scrapes, and bug-bites since 1884.
Your concern with the compounds emitted by wood chips has less to do with phenol per se and more to do with the sap, tar, or resin that is contained in the wood. Many of the compounds in plant-based goos are in the class of chemical compounds called terpenes, and while a little bit of terpene, like the essential oil from hops or lemons, can impart a nice flavor to your favorite beverage, a swig of turpentine (pine tar terpenes) can be toxic.
Since alders are in the same family as birches and hazelnuts (Betulaceae), the sap is more likely to be able to be made into a beverage (birch beer) than to be distilled into turpentine, and so it is probably safe for your chickens and rabbits. Of course, any animal can have an allergic reaction to almost any natural product chemical under the sun, so you would want to add it in little by little to the bedding or litter and watch for any adverse reactions.
(Note: John Elliott: please correct any of the below if I am off on something...)
I was actually waiting for someone with a stronger "organic chemistry" background than I to respond first, and we are very lucky to have John to expand and explain such things so thoroughly.
I can speak to this subject from an "animal husbandry" perspective.
Avid culture perspective. I know as a zookeeper, and as someone that has designed a few Aviaries, that we are guarded against any "monolithic" bedding mix unless well understood. Cedar shavings, for example are "safe" in some applications, and with some species, while with others can be very toxic. It also depends on the age of the shavings and the actual type of cedar they came from. Young birds, and reptiles for example can be killed in less than 24 hours with exposure to these volatile oils.
Birch, Alder, Maple, shavings could be safe if dry, and well aged or could become reactive, as John has suggested. I think it would depend on other factors besides just it species. Such as shaving type (i.e. chip or true shaving) as well as the period of time between removing them from an enclosure after they are "soiled" by the birds.