<< Chapter < Page Chapter >> Page >

Lab 3: grignard reaction

Objective

The purpose of this laboratory exercise is to perform a classic method for the synthesis of secondary alcohols: the addition of a Grignard reagent to an aldehyde (other than formaldehyde).

Background information

After the most remarkable discovery by Victor Grignard in 1900, organometallic (An organic compound containing a metal atom directly attached to a carbon) compounds are very useful in organic synthesis. This discovery changed the course of organic chemistry and earned him the Nobel Prize in 1912. We now refer to such compounds as Grignard Reagents.

The Grignard reaction is one of the characteristic reactions of carbonyl compounds. It is especially useful as a means of forming new carbon-carbon bonds, something that we haven't seen much of until now. In this type of reaction a C-M bond is present (C=carbon and M=metal) which is covalent in nature. The metal is usually magnesium but lithiummore reactive and is also used. The more polar the C-M bond is, the more will be its reactivity. The reactivity order is known from electrochemical series:

M= Li>K>Ca>Na>Mg>Al>Zn>Fe>Sn etc

Traditionally alkyl magnesium halides are known as Grignard reagent (R-Mg-X).

All kinds of alkyl halides react (Iodides are more reactive than bromides which in turn are more reactive than chlorides) and amazingly, even bromobenzene and other aryl bromides and iodides react easily with magnesium. This is particularly surprising since the aromatic halogen is so unreactive. For example, it is inert to refluxing with aqueous sodium hydroxide at temperatures in excess of 200˚C. The unshared pair of electrons on ether oxygen complexing with the Mg is believed to contribute to the stability of the reagent. Note: Aryl halides (ArX) and vinyl halides that are inert to nucleophilic substitution are reactive with this Grignard reaction.

Preparation and mechanism of grignard reagent:

The mechanism of Grignard reagent (GR) is not clear till today, but it is believed that the reaction is taking place on the metal surface. Since GR does not react with aprotic solvents (e.g. ether, THF etc), these solvents are widely used. The mechanism of formation of GR as follows: it goes via one electron transfer, followed by rapid combination of organic group with the metal center. From the mechanistic point of view, carbon-bromine bond should be broken prior to the reaction with magnesium.

Solutions of some Grignard reagents such as methylmagnesium bromide, ethylmagnesium bromide, and phenylmagnesiumbromides are commercially available. Here is an example of Grignard reaction which explains the effect of solvent.

The Grignard reagent is made from the direct reaction of magnesium with an alkyl halide.

The key to the function of the Grignard reagent is the reversal of the normal polarity of bonds to carbon (This type of reversal of polarity in carbon center is known as Umpolung). Because magnesium is more electropositive than carbon the carbon acquires a δ size 12{δ - {}} {} charge whereas carbon, when it has a charge at all, usually has δ + size 12{δ+{}} {} charge from bonding to halogens, oxygen, and nitrogen.

Questions & Answers

what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
Bharti
do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
Damian Reply
absolutely yes
Daniel
how to know photocatalytic properties of tio2 nanoparticles...what to do now
Akash Reply
it is a goid question and i want to know the answer as well
Maciej
characteristics of micro business
Abigail
for teaching engĺish at school how nano technology help us
Anassong
Do somebody tell me a best nano engineering book for beginners?
s. Reply
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
s.
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
Tarell
what is the actual application of fullerenes nowadays?
Damian
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
Tarell
what is the Synthesis, properties,and applications of carbon nano chemistry
Abhijith Reply
Mostly, they use nano carbon for electronics and for materials to be strengthened.
Virgil
is Bucky paper clear?
CYNTHIA
so some one know about replacing silicon atom with phosphorous in semiconductors device?
s. Reply
Yeah, it is a pain to say the least. You basically have to heat the substarte up to around 1000 degrees celcius then pass phosphene gas over top of it, which is explosive and toxic by the way, under very low pressure.
Harper
Do you know which machine is used to that process?
s.
how to fabricate graphene ink ?
SUYASH Reply
for screen printed electrodes ?
SUYASH
What is lattice structure?
s. Reply
of graphene you mean?
Ebrahim
or in general
Ebrahim
in general
s.
Graphene has a hexagonal structure
tahir
On having this app for quite a bit time, Haven't realised there's a chat room in it.
Cied
what is biological synthesis of nanoparticles
Sanket Reply
what's the easiest and fastest way to the synthesize AgNP?
Damian Reply
China
Cied
types of nano material
abeetha Reply
I start with an easy one. carbon nanotubes woven into a long filament like a string
Porter
many many of nanotubes
Porter
what is the k.e before it land
Yasmin
what is the function of carbon nanotubes?
Cesar
I'm interested in nanotube
Uday
what is nanomaterials​ and their applications of sensors.
Ramkumar Reply
what is nano technology
Sravani Reply
what is system testing?
AMJAD
preparation of nanomaterial
Victor Reply
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
Privacy Information Security Software Version 1.1a
Good
Got questions? Join the online conversation and get instant answers!
QuizOver.com Reply

Get the best Algebra and trigonometry course in your pocket!





Source:  OpenStax, Chem217labsfall07. OpenStax CNX. Oct 16, 2007 Download for free at http://cnx.org/content/col10463/1.4
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Chem217labsfall07' conversation and receive update notifications?

Ask