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From Wikipedia

Reaction mechanism

In chemistry, a reaction mechanism is the step by step sequence of elementary reactions by which overall chemical change occurs.

Although only the net chemical change is directly observable for most chemical reactions, experiments can often be designed that suggest the possible sequence of steps in a reaction mechanism. Recently, electrospray ionization mass spectrometry has been used to corroborate the mechanism of several organic reaction proposals.

Description

A chemical mechanism describes in detail exactly what takes place at each stage of an overall chemical reaction (transformation). It also describes each reaction intermediate, activated complex, and transition state, and which bonds are broken (and in what order), and which bonds are formed (and in what order). A complete mechanism must also account for all reactants used, the function of a catalyst, stereochemistry, all products formed and the amount of each, and what the relative rates of the steps are. Reaction intermediates are chemical species, often unstable and short-lived, which are not reactants or products of the overall chemical reaction, but are temporary products and reactants in the mechanism's reaction steps. Reaction intermediates are often free radicals or ions. Transition states can be unstable intermediate molecular states even in the elementary reactions. Transition states are commonly molecular entities involving an unstable number of bonds and/or unstable geometry which may be at chemical potential maxima.

The electron or arrow pushing method is often used in illustrating a reaction mechanism; for example, see the illustration of the mechanism for benzoin condensation in the following examples section.

A reaction mechanism must also account for the order in which molecules react. Often what appears to be a single step conversion is in fact a multistep reaction.

Examples

Consider the following reaction:

CO + NO2→ CO2 + NO

In this case, it has been experimentally determined that this reaction takes place according to the rate law R = k[NO_2]^2. Therefore, a possible mechanism by which this reaction takes place is:

2 NO2→ NO3 + NO (slow)
NO3 + CO → NO2 + CO2 (fast)

Each step is called an elementary step, and each has its own rate law and molecularity. The elementary steps should add up to the original reaction.

When determining the overall rate law for a reaction, the slowest step is the step that determines the reaction rate. Because the first step (in the above reaction) is the slowest step, it is the rate-determining step. Because it involves the collision of two NO2 molecules, it is a bimolecular reaction with a rate law of R = k[NO_2]^2. If we were to cancel out all the molecules that appear on both sides of the reaction, we would be left with the original reaction.

In organic chemistry, one of the first reaction mechanisms proposed was that for the benzoin condensation, put forward in 1903 by A. J. Lapworth.

Modelling

A correct reaction mechanism is an important part of accurate predictive modelling. For many combustion and plasma systems, detailed mechanisms are not available or require development.

Even when information is available, identifying and assembling the relevant data from a variety of sources, reconciling discrepant values and extrapolating to different conditions can be a difficult process without expert help. Rate constants or thermochemical data are often not available in the literature, so computational chemistry techniques or group-additivity methods must be used to obtain the required parameters.

At the different stages of a reaction mechanism's elaboration, appropriate methods must be used.

Molecularity

Molecularity in chemistry is the number of colliding molecular entities that are involved in a single reaction step.

  • A reaction involving one molecular entity is called unimolecular.
  • A reaction involving two molecular entities is called bimolecular.
  • A reaction involving three molecular entities is called termolecular.


From Yahoo Answers

Question:1) How would these compounds be arranged in order of increasing reactivity towards attack of the Grignard reagent at the carbonyl carbon atom: methyl benzoate, benzoic acid, benzaldehyde, acetophenone, benzoyl chloride. Explaining the basis for each. 2) What is/are the product(s) of reaction of each of the above carbonyl containing compounds with an excess of a Grignard reagent, RMgBr? 3) How might primary, secondary, and tertiary alcohols be prepared from a Grignard reagent, RMgBr, and a suitable carbonyl-containing compound? What would be the chemical equations, with stoichiometry, for these preparations?

Answers:benzoyl chloride's pretty good. no acidic hydrogens whatsoever methyl benzoate. a good ester, but some weakly acidic hydrogens benzaldehyde. hydrogen on the aldehyde is a poor acid, acetophenone, alpha hydrogen can be deprotonated instead benzoic acid. no question, the grignard will simply take the acid hydrogen and not attack the carbon at all respectively, in the above order lose the Cl, add the R lose the methoxide, add R to make ketone, then another R will attack the carbonyl, adding on, and making the ketone a tertiary alcohol add R, make aldehyde a secondary alcohol again, secondary alcochol, with an extra R you'd get benzoate(-) and MgBr(+), along with a full alkane R. ruined grignard primary, react with formladehyde secondary, react with aldehyde tertiary, react with ketone/ester derivative

Question:How do Grignard reagents react with alkenes, alkynes and haloalkanes? For example, how would ethyl magnesium bromide react with t-butyl bromide and propyne respectively? Also, what would happen if EtMgBr reacted with 1-bromobut-2-ene? Any clues would be much appreciated!

Answers:Grignard reagents act as nucelophiles and do not react with the unsaturated bonds of alkenes and alkynes. Halogenoalkanes react with magnesium to form Grignard reagents of the form RMgX. Adding another halogenoalkane into the reaction mixture will just form another Grignard reagent R'MgX. In the reaction of 1-bromobut-2-ene with RMgX the bromine atom is replaced by the R group.

Question:The Grignard reagent is generated using bromobenzene as the alkyl halide, then reacted immediately with benzophenone producing triphenyl carbinol, a 3 substituted alcohol. Write an equation showing what happens if the apparatus isn't dry and the Grignard reagent reacts with water. Besides water, what is being removed from the ether layer when it is washed with a saturated NaCl solution? What is the purpose of washing the crude product with petroleum ether before filtration?

Answers:Lancenigo di Villorba (TV), Italy YOUR SYNTHESIS Your Chemistry's Teacher start from manipulate an Ether's Volume in a glass-beaker where he dissolved BromoBenzene obtaining a clear solution. WARNING!! Ether is a very flammable liquid!! DON'T STORE IT NEIGHBOUR HEAT's SOURCES!! DON'T BREATH ITS VAPOUR!! DON'T LET IT ON AIR's EXPOSURE!! Oxygen's air may dissolve in Ether converting it in a Least Stable Liquid. Now, he put some Metallic MAGNESIUM PIECEs in the vessel, so THE MIXTURE REACTS FASTLY AS THE METAL DISAPPEARS Mg(s) + C6H5Br(eth.) ---> C6H5MgBr(eth.) When the liquid back on its Quiet appearance, the Grignard's Reactant is ready to react further : he add the Substrate as BenzoPhenone, so THE FOLLOWING AND MAIN REACTION MAY OCCURs : (C6H5)C=O(eth.) + C6H5MgBr(eth.) ---> ---> (C6H5)3CHOMgBr(eth.) WHAT IT IS HAPPENED? The Grignard's Reaction carried out the MAGNESIUM SALT's TRI-PHENYLCARBINOL. MOIST VESSELs What does it occur if the Glass-Vessel was a moist one? Water present in the Vessel may react against Metallic Magnesium Mg(s) + 2 H2O(aq) ---> Mg(OH)2(s) + H2(g) You could see several gas bubbles, e.g. Hydrogen ones, instead Grignard's Reactant. NaCl's SOLUTION I overwritten that Glass-Vessel have to be a STRICTLY DRY ONE. On the other hand, the Chemistry's Teacher will use a NaCl's SATURATED SOLUTION in order to rinse the Ether Mixture when the Grignard's Reaction just finished. Hence, in a Separatory Funnel, it put the Ether Mixture and he add the Petroleum Solvent ; few minutes later, he recover the Petroleum Solvent where TRI-PHENYLCARBINOL stands. As it is SODIUM SALT SATURATED SOLUTION, this aqueous liquid ACTS BY MEANS OF AN ACID-BASE EVENT (C6H5)3CHOMgBr(eth.) + H2O(aq) ---> ---> (C6H5)3CHOH(eth.) + Mg(OH)Br(eth.) so the Ether Mixture may retain Magnesium Compound as "Lewis Electronic Dot's Theory" may explain ; on the other hand, Aqueous Mixture cannot gain Magnesium's Salts as it results SATURATED One. PETROLEUM RINSING The Chemistry's Teacher could proceed to filtration but he prefer to remove the Magnesium's Salts present in the Ether Mixture. Since Magnesium Salts aren't soluble in Petroleum Solvent then Chemistry's Teacher uses these solvents in order to recover TRI-PHENYLCARBINOL away from Magnesium Compounds. Furthermore, in a Separatory Funnel, it put the Ether Mixture and he add the Petroleum Solvent ; few minutes later, he recover the Petroleum Solvent where TRI-PHENYLCARBINOL stands. Finally, this latter liquid undergoes filtration so retaining TRIPHENYLCARBINOL upon the Filter-Paper. I hope this helps you.

Question:

Answers:RMgBr (Grignard reagent ) + CH3COCl (Acid chloride) --> R2CHOH (TERTIARY ALCOHOL) First, Grignard reagent will react with acid chloride to form a ketone (CH3COR), Excess Grignard reagent will attack the carbonyl group of ketone again to form a tertiary alcohol which consist two identical alkyl part from Grignard reagent. The mechanism is the same as the reaction between ester and Grignard reagents. http://www.fsj.ualberta.ca/chimie/chem161/bACIDS/sld077.htm

From Youtube

Grignard Reaction :After making a Gringard Reagent, we were then synthesizing an alcohol (triphenylmethanol). The actual alcohol is in solid form caked to the sides of the flask. If you notice, the hot plate is off, it is only stirring a small stir bar inside the flask. Also, you might be able to see condensation occurring towards the top of the flask. There is a condenser running cool water on the outside of the glass directly above the round bottomed flask. This is called an exothermic reaction ZOMGLOLSTICKS

Grignards (1) :Organic chemistry: How to make Grignard reagents and alkyl lithiums (organometallics). Reactions of Grignards and alkyl lithiums (with protic solvents, aldehydes and ketones, and epoxides/oxacyclopropanes). Synthesis problems using radical halogenation, E2, SN2, oxidation (PCC), and Grignards for synthesis. Theretrosynthesis technique for solving synthesis problems. This is a recording of a tutoring session, posted with the students' permission. These videos are offered on a "pay-what-you-like" basis. You can pay for the use of the videos at my website: www.freelance-teacher.com For the printable "handouts" discussed in these videos, go to my website. For a list of all the available video series, arranged in suggested viewing order, go to my website. For a playlist containing all the videos in this series, click here: www.youtube.com (1) Radical halogenation for synthesis (2) Continued (3) E2 for synthesis (4) SN2 and oxidation (PCC) for synthesis. Retrosynthesis (5) Continued (6) How to make a Grignard or alkyl lithium (7) Continued. Reactions of Grignards--with protic solvents, ketones, or aldehydes (8) Continued (9) Continued. Synthesis using Grignards (10) Continued. Retrosynthesis (11) Reaction of Grignards with oxacyclopropanes (epoxides) (12) Continued. Summary of Grignards and synthesis techniques