Aldehydes by: ajfcalapatia, cjoca and amvitug

Aldehydes  

- are a class of highly reactive organic chemical compounds that contain a carbonyl group (in which a carbon atom is double bonded to an oxygen atom) and at least one hydrogen atom bound to the alpha carbon (the central carbon atom in the carbonyl group). 
- The word aldehyde was coined by Justus von Liebig as a contraction of the Latin alcohol dehydrogenatus.

The base form of an aldehyde, A planar carbon center that is connected
by a double bond to oxygen and a single bond to hydrogen.

 Formaldehyde (Methanal),
the simplest aldehyde.

Uses of aldehydes:


Hundreds of individual aldehydes are used by chemists daily to synthesize other compounds, but there are less important in industrial synthesis (that is, the production of compounds on a scale of tons.) Only one aldehyde, formaldehyde, is used to a significant degree in industry worldwide (as it is used to preserve the dead, as an insecticide and used to create plastic), as determined by number of tons of the chemical utilized per year.  


Other Examples of Aldehydes:

• Acetaldehyde (Ethanal) - CH₃CHO or MeCHO

• Propionaldehyde (Propanal) - CH₃CH₂CHO

• Butyraldehyde (Butanal) - CH₃(CH₂)₂CHO

Reaction Mechanism by: ajfcalapatia & jngaspar

          A Reaction Mechanism is the complete description of how the reactants are converted to products.


          A reaction mechanism is presented as a series of single-step changes called elementary process or reaction which expresses how molecules or ions react with each other.

          Based on numbers of molecules involved in the elementary step, there are three kinds of elementary steps:

1.  Unimolecular step (or process)

 - an elementary process that involves the decomposition of a single molecule or ion.

 - is always a first order reaction

(1) O₃ ---> O₂ + O

or in general,

A = B + C + D, Rate = k [A]
A* = X + Y, Rate = k [A*] 

A* represents an excited molecule. 

Rate = k [O₃]

2. Bimolecular process 

 - an elementary process in which 2 molecules or ions collide.

(2) O₃ + O ---> 2O₂

or in general,

A + A = B + C, Rate = k [A]²
A + B = X + Y, Rate = k [A] [B]

  

Rate = k [O₃] [O]

3. Trimolecular/Termolecular Process

- simultaneous collision of three molecules.

 (3) O + O₂ + N₂ = O₃ + N₂

or in general,

A + A + A = products, Rate = k [A]³

A + A + B = products, Rate = k [A]² [B]

A + B + C = products, Rate = k [A] [B] [C]

Rate = k [O] [O₂] [N₂]

 Rate-Determining step 

 - is the slowest step in a chemical reaction.

 - determines the rate of overall reactions.

Many reactions occur in a number of elementary steps.

ex.

H₂+ 2Cl ---> I₂ + 2HCl

where the rate law is

Rate = k [H₂] [ICl]

The problem is to postulate a reaction mechanism which will confirm to the rate law and produce the net reaction.

                    (1) Slow: H₂ + ICl   --->   HI + HCl

                    (2) Fast:  HI  + ICl   --->   I₂  + HCl

                          Net:   H₂ + 2ICl  --->   I₂  + 2HCl

The rate law expressions for the two elementary steps are

                    Rate (1) = k₁ [H₂] [Cl]          and          Rate (2) = k₂ [HI] [ICl]

Since the rate law of original equation is also the rate law of equation (1) in the reaction mechanism, then it is the slow step or the rate determining step.

Reaction intermediate/product

- the specie that is formed in one step of the reaction mechanism and consumed in a subsequent step.

- does not appear in the net equation for the overall.

Catalyst

- the specie that is introduced as a reactant and later regenerated without change in the subsequent step.

- like a reactant product it also does not appear in the net equation for the overall.

EXAMPLE 1

A mechanism proposed for the reaction between SnCl₂ and FeCl₃ is as follows.

                    SnCl₂ + 2Cl   --->   SnCl₄

                    SnCl₄ + FeCl₃   --->   SnCl₃ + FeCl₂ + 2Cl

                    SnCl₃ + 2Cl₃   --->   SnCl₄ + FeCl₂

a. Write the overall chemical reaction.

b. What are the reaction intermediates of the reaction?

c. What specie(s) serve the catalyst?

Solution:

a. The overall reaction is the sum of the three equations

                    SnCl₂ + 2Cl   --->   SnCl₄

                    SnCl₄ + FeCl₃   --->   SnCl₃ + FeCl₂ + 2Cl

                    SnCl₃ + FeCl₃   --->   SnCl₄ + FeCl₂             

                    SnCl₂ + 2FeCl_{3   --->}   SnCl_{4 +} FeCl₂

b. The intermediate products or reaction intermediates are the specie(s) formed in one elementary step and consumed in a subsequent step. So the intermediates are SnCl_4 and SnCl_3.

c. The catalyst is Cl because it is introduced as a reactant and later regenerated without change in the subsequent step.

Sources:

     Breaking Through Chemistry

          by: Saranay M. Baguio & Rose Mary B. Butaran

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