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Chemistry of mint

Page history last edited by Fatima Al-Zahraa Itani 8 years, 11 months ago

Chemistry of Mint
   

Uncovering the mystery of mint as well as any other matter should start with a chemical study to find out its composition.This chemical minty page will inform you about the composition of mint and the chemistry of its main component : MENTHOL

 

I- Chemical Composition of MINT 

 

The chemical components of peppermint oil are menthol, menthone, cineole, methyl acetate, methofuran, isomenthone, limonene, b-pinene, a-pinene, germacrene-d, trans-sabinene hydrate and pulegone. Menthol in mint is responsible for the cooling effect of mint.

 

 

 

 

 

II- Chemistry of MENTHOL

 
A- Molecular formula of Menthol:  C 10 H 20

B- Structural formula of Menthol:

                                         

                                          

 

 

 

 

 

 

 

 

 

 

 

C-  Functional groups of Menthol: In the natural compound, the isopropyl group is in the trans orientation to both the methyl and hydroxyl  groups.
 

 D- Reactions: Menthol reacts in many ways like a normal secondary   alcohol.     It  is  oxidised to menthone  by oxidising agents such as chromic acidor dichromate,though under some conditions the oxidation can go further and break open the ring. Menthol is easily dehydrated to give mainly 3-menthene, by the action of 2% sulfuric acid. PCl5 gives menthyl chloride.

 

 

       

Reactions of menthol

 

  III- Physical Properties of MENTHOL

     

Molecular formula

C10H20O

Molar mass

156.27 g/mol

Appearance

White or colorless
crystalline solid

Density and phase

 

0.890 g/cm3, solid

(racemic or (−)-isomer)

Solubility in water

Slightly soluble, (−)-isomer

In ethanol, diethyl ether, acetone, chloroformacetic acid, hexane

 

Soluble

Melting point

36-38 °C (311 K), racemic
35-33-31 °C, (−)-isomer

Boiling point

212 °C (485 K)

Odor

Peppermint oil

Density

0.89 g/l

 

Vapor Pressure (mm Hg)

0.8 at 21C (70F)

 

Chiral rotation *D

-50° at 18 °C, 10% Ethanol Solution

 

Main Hazards

Irritant, flammable

 

Thermodynamic Data

Phase behavior : Solid, liquid, gas

Related Alcohols

Cyclohexanol, Pulegol,
Dihydrocarveol, Piperitol

Related Compounds

Menthone, Menthene, Thymol,

 p-Cymene,
Citronellal

Note: The data are given for materials in their standard state (at 25°C, 100 kPa).

 

 

 

   

 

 

 

 

 

 

    

 

      

 

IV- 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.

                                                     

 

If you noticed the letters (L) and (D) before the name of the stereoisomers  in the text but in the figure the names are preceeded by (-) or (+). Don't get confused, it's simple.

(L) means levorotatory (D) means dextrorotatory. The (L) stereoisomer rotates the light to the left and it can be symbolized by (-);however, the (D) stereoisomer rotates the light to the right and it can be symbolized by (+). (+) and (-)menthol are mirror images and not superimposable , thus they are called enantiomers.

Stereoisomers of any substance as well as menthol have different properties.

However, the enantiomeric isomers  (d  and l)have identical physical properties (apart from their specific rotation), but the racemates ( racemic mixture: dl) differ from the optically active forms in, for example, their melting points.

 

 

Note: A racemic mixture composed of D-isomer and its mirror image L-isomer each of 50%. That is, it is a mixture of enantiomers. This mixture is not able to rotate light, thus it is optically inactive.

 

The table below shows the melting point, boiling point and angle of rotation of polarized light in 20% ethyl alcohol solution of one optically active and the racemic form each stereoisomer of menthol.

 

    

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? 
 

Question Mark Clip Art. That's when

 

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: 

Chirality of Molecules

   

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