Note: The data are given for materials in their standard state (at 25°C, 100 kPa).
------------------------------------------------------------III- Stereochemistry of MENTHOL: The Menthol molecule contains 10 carbon atoms, 3 of these carbon atoms are chiral (that is bonded to 4 different groups of atoms). This results in 8 stereoisomers for Menthol which are: (L)-menthol, (D)-menthol , (L)-isomenthol, (D)-isomenthol, (L)-Neomenthol, (D)-Neomenthol, (L)-Isoneomenthol and (D)- Isoneomenthol.
Stereoisomers are isomers which have their atoms connected in the same sequence but differ in the way the atoms are oriented in space - i.e. the difference between two stereoisomers lies only in the three dimensional arrangement of atoms. The follow figure shows the condenesed structural formulas of the 8 stereoisomers of menthol. The straight line bonds are lining in the plane of this page. The dark bonds are lining toward you and the dash bonds are lining away backward. |
Stereoisomer |
Melting point oC |
Boiling point at 760 Mm, oC |
(α) in 20% ethyl alcohol |
l- Menthol |
42.0 |
212 |
-50.0 |
d- Menthol |
43.0 |
216.5 |
+ 50.0 |
dl- Menthol |
38.0 |
216.5 |
0 |
d-Neo-Menthol |
-15.0 |
211.7 |
+20.0 |
dl-Neo-Menthol |
52.0 |
211.7 |
0 |
d-Iso-Menthol |
82.5 |
218.6 |
+26.0 |
dl-Iso-Menthol |
63.5 |
218.6 |
0 |
d-Neo-Iso-Menthol |
-8.0 |
214.6 |
+2.0 |
dl-Neo-Iso-Menthol |
13.5 |
214.6 |
0 |
So Which Stereosomer Of Menthol Is Responsible For The Fresh Cooling Effect Of Mint?
Before answering this question, let’s have an overview for some physical properties of the 8 stereoisomers of menthol in the following table
Characteristics of Each Stereoisomer of Menthol |
|
(-) neomenthol |
(+) neomenthol |
Minty - musty – fresh – earthy campherous – some cooling Taste threshold = 600 ppb Cooling threshold = 25000 ppb |
Minty - musty – fresh – some cooling - sweet Taste threshold = 500 ppb Cooling threshold = 2500 ppb |
(-) menthol |
(+) menthol |
Very cooling – sweet – fresh - minty Taste threshold = 400 ppb Cooling threshold = 800 ppb |
Fresh – cooling – minty with musty – bitter Taste threshold = 300 ppb Cooling threshold = 3000 ppb |
(-) isomenthol |
(-) isomenthol |
Musty - sweet– hay – earthy camphoraceous – herbaceous – slight cooling. Taste threshold = 600 ppb Cooling threshold = 30000 ppb |
Musty – woody – fresh – carrot minty - earthy camphoraceous –slight cooling. Taste threshold = 700 ppb Cooling threshold = 7000 ppb |
(-) neoisomenthol |
(+) neoisomenthol |
Minty – earthy – woody – camphoraceous – sweet – minty – slight cooling Taste threshold = 1000 ppb Cooling threshold = 6000 ppb |
Minty – earthy – woody – camphoraceous – carrot – minty – herbaceous - very little cooling Taste threshold = 200 ppb Cooling threshold = 25000 ppb |
Note: ppb means part per billion
If you compared the cooling threshold of the stereoisomers of menthol you can find the answer.
Yes it is (-) or (L) menthol who has the lowesr cooling threshold and is responsible for the cooling effect of pepper mint.
To Know more About Stereochemisty and chirality You Can view this work sheet by just clicking on this link
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