  The so-called "unidentified infrared emission" (UIE) features at 3.3, 6.2,
7.7, 8.6, and 11.3 micron ubiquitously seen in a wide variety of astrophysical
regions are generally attributed to polycyclic aromatic hydrocarbon (PAH)
molecules. Astronomical PAHs often have an aliphatic component (e.g., aliphatic
sidegroups like methyl --CH3 may be attached as functional groups to PAHs) as
revealed by the detection in many UIE sources of the aliphatic C--H stretching
feature at 3.4 micron. With its unprecedented sensitivity, unprecedented
spatial resolution and high spectral resolution, the James Webb Space Telescope
(JWST) holds great promise for evolutionizing the studies of aliphatics and
aromatics in the universe. To facilitate analyzing JWST observations, we
present a theoretical framework for determining the aliphatic fractions
(\eta_ali) of PAHs, the fractions of C atoms in aliphatic units, from the
emission intensity ratios of the 3.4 micron feature to the 3.3 micron feature.
To demonstrate the effectiveness of this framework, we compile the 3.3 and 3.4
micron UIE data obtained in the pre-JWST era for an as complete as possible
sample, and then apply the framework to these pre-JWST data. We derive a median
aliphatic fraction of <\eta_ali>=5.4\% and find that the aliphatic fractions
are the highest in protoplanetary nebulae illuminated by cool stars lacking
ultraviolet radiation. Nevertheless, the "hardness" of stellar photons is not
the only factor affecting the PAH aliphaticity, other factors such as the
starlight intensity may also play an important role.
