Drawing Lewis Structures of Alkanes
Python 2, 620 bytes
import re
i=input()
s='m|e|pr|b|p|hex|h|o|n|de';d=dict(zip(s.split('|'),range(1,11)))
z=[[eval('['+a+']'),d[b]]for a,b in re.findall('(?:(\d[,\d]*).*?[\-ia]|l)('+s+')',i[:-3])]
v=z[-1][1]
l=[[0,0]for _ in range(v)]
c=0
for a,b in sorted([(i,b)for a,b in z[:-1]for i in a]):l[a-1][c]=b;c=~c
m=[max(x) for x in zip(*l)]
L,R=[[[' 'for _ in '_'*2*i]for _ in '_'*(2*v+1)]for i in m]
c=[' |'*v+' ']
C=c+['-C'*v+'-']+c
for i in range(len(l)):
 X=L;q=2*i+1
 for a in l[i]:
  if a&gt;0:
   for j in range(0,2*a,2):
    X[q][j]='C'
    X[q-1][j]=X[q+1][j]='-'
    X[q][j+1]='|'
  X=R
for l in zip(*L)[::-1]+C+zip(*R):print ''.join(l)


Explanation
Input:
'2-ethyl-2,4,6-trimethyldecane'
First parses to string with regex (last group is root):
[[[2], 2], [[2, 4, 6], 1], [[], 10]]
Each branch is written in an array of length len(root) (Alternating up/down is handled here):
[[0, 0], [1, 2], [0, 0], [1, 0], [0, 0], [0, 1], [0, 0], [0, 0], [0, 0], [0, 0]]
'Left' and 'Right'(L,R) and 'root' (C) string branches are initialized.
Each branch is then added to the corresponding 'string' branch (big loop).
The two sides and center are printed at the end:
   |   |             
  -C- -C-            
 | | | | | | | | | | 
-C-C-C-C-C-C-C-C-C-C-
 | | | | | | | | | | 
  -C-     -C-        
   |       |         
  -C-                
   |