Ask A Scientist© Chemistry Archive

name         John C.
status       student
age          18

Question -   My group in chemistry has to make a presentation of a
chapter which is titled, "Addition Reactions of Alkenes and Alkynes."  As
we have read
through the information we are a little confused about saturated and
unsaturated hydrocarbons and how Alkenes and Alkynes go together.  I do
understand that they are talking about the addition of a variety of acids,
halogens, water, and hydrogen.  I am just confused when they talk
about adding bromine or some other element to an unsaturated compound how
do Alkenes and Alkynes become involved in it.  If you know any simple
terms to explain this information I would love to hear back from you. If
you need more information to help make this clearer let me know.
-----------------------------
John,

If as you say, "I do understand that they are talking about the addition of a
variety of acids, halogens, water, and hydrogen," it appears you have mastered
the difficult before the easy.

In general, the reactions are referred to as electrophilic additions because
the attacking species is something that is electron seeking -- something 
with a
(+) charge, or something that is otherwise electron deficient.

Consider the unsaturated compound, ethene (C2H4): The Pi electrons in the
double bond are vulnerable to attack by electron-seeking species. Bromine 
(Br2)
molecules when brought into the vicinity of a double bond can be polarized 
into
what can be described as a nascent  Bromonium and Bromide ion pair. Bromonium
is a (+) charged ion, bromide is (-). If the bromonium ion takes a pair of
electrons from the Pi bond to form a bond with either carbon atom in the
molecule, the result is a three-membered cyclic intermediate wherein the 
Br+ is
bridging the two (formerly doubly bonded) carbon atoms. The Br(-) ion then
attacks the backside of the C-C bond to complete the reaction. The result is a
pair of adjacent carbon atoms each bearing a bromine atom bonded to it -- the
compound is 1,2-dibromoethane.

A somewhat similar process occurs with alkynes. In the case of ethyne (C2H2),
two molecules of Br2 can add across the triple bond in that molecule. The
result is the compound, 1,1,2,2, tetrabromoethane.

In situations where HCl, HBr, HOCl, H2SO4, H2O, are the things to be added, in
each case, the addend splits into a (+) and (-) species -- of which, the (+)
part then adds to the unsaturated center by acquiring a pair of electrons from
the Pi bond. These are examples of electrophilic additions of a more
straightforward type because they involve carbocations.

Addition of H2 is a somewhat different case from either of the above. 
Catalysts
are involved and the reaction mechanism is quite different from those above.

All this would be much easier to explain if I were able to use diagrams. I 
urge
you and your group to look in almost any introductory college level chemistry
book. I am sure you will find a very nice explanation of all this (with 
detailed
diagrams) therein.

Regards,
ProfHoff
=========================================================
Alkenes and alkynes are two classes of unsaturated compounds. Alkenes
contain a carbon-carbon double bond; alkynes contain a carbon-carbon triple
bond. Although the mechanism is slightly different, adding HBr across a
double bond is similar in effect to adding Br2 across a double bond. One
atom of the diatomic ends up on one of the formerly double-bonded carbons,
and the other atom from the diatomic goes on the other carbon. So,

   H2C=CH2 + HBr --> H3C-CH2Br

   H2C=CH2 + Br2 --> H2BrC-CH2Br.

In the reactions drawn above, the alkene ethylene (aka ethene) is involved.

Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois
=========================================================
Alkanes are hydrocarbons in which all of the carbon atoms are connected by
single carbon-carbon bonds. Alkenes are hydrocarbons in which at least one
pair of carbon atoms is connected by a double carbon-carbon bond. Alkynes
are hydrocarbons in which at least one pair of carbon atoms is connected by
a triple carbon-carbon bond.
Ethane, ethylene [ethene], and acetylene [ethyne] are the simplest examples.

In the presence of certain reagents, the ones you have mentioned above, a
double carbon-carbon bond can be converted to a single carbon-carbon bond by
adding an atom of the reagent to each of the carbons forming the double
bond. Likewise, a triple bond can be converted to a double bond by adding an
atom of the reagent to each carbon atom of the triple bond. And depending
upon the reagent this product may react further with the reagent to add two
atoms and form a doubly substituted alkane.

These reactions have been studied extensively, but going into the details of
which reagents do what is beyond the scope of a forum such as this. If you
wish more details you can consult most any introductory college organic
chemistry text.

Vince Calder
=========================================================
Hi John!
As you know the organic compounds in which some carbon atoms share more
than 2 electrons with another atom,  form multiple covalent bonds. So the
alkenes are hydrocarbons whose molecules contain a carbon-carbon double
bond,
(share 4 electrons), and alkines contain a triple bond (share 6 electrons).
Addition reactions are the most chracteristic of compounds that contain
multiple
bonds. When a reagent X-Y adds to a carbon-carbon double bond, X becomes
attached to one atom of the double bond and Y to the other, and the double
bond
changes to a single one.Similarly with X-X reagent as one bromine molecule)
Because the alkines have  a triple bond, there is the possibility that 2
moles of the
adding reagent to react..In the case you mention, 2 molecules of bromine
will
react ( under the proper experimental conditions), breaking the triple bond
and producing  a new compound with 2 bromine atoms added to each carbon
that formed the triple bond. Also under controlled conditions, the addition
can be partial, in that case a double bond remains and 1 bromine molecule
will react, and each of the 2  bromine atoms
becomes attached to each C that forms that double bond.
Thanks for asking NEWTON!
Mabel
(Dr. Mabel Rodrigues)
=========================================================
John :

         An alkene refers to a hydrocarbon compound having a carbon-carbon
double bond, while an alkyne is a hydrocarbon compound having a
carbon-carbon triple bond. Any carbon-carbon bond which is either double or
triple is called unsaturated, so that both alkenes and alkynes are
unsaturated hydrocarbons. These bonds are called unsaturated because you
can break the double or triple bond by adding a functional group to one or
both of the carbon atoms, by an addition reaction, thereby making all of
the bonds to the carbon atoms single bonds. Once all of the carbon bonds
are single bonds, you can't add anything else to them, and then they are
saturated. You see ?

         In my own work, we needed a chemical compound that would remove
hydrogen gas from an enclosed container. We used a hydrocarbon compound
that contains carbon-carbon triple bonds. The hydrogen atoms added
themselves to the molecule, breaking the triple bonds, forming
carbon-carbon double bonds

                            -------
------
                      ------    C ------- C
------  + H2      ---------------> ------ CH ------ CH -----
                            -------


Then, more hydrogen would combine itself with the molecule, breaking the
double bonds

                              --------
                      ------    CH ------- CH
------  + H2      ---------------> ------ CH2 ------ CH2 -----


We were then left with a saturated hydrocarbon, which could not accept any
more hydrogen. Your additions reactions will work in just this way. The
simplest addition reactions are those which add singly-charged ions, such
as hydrogen, to an unsaturated hydrocarbon. Other singly-charged ions which
work quite well are chlorine, bromine, iodine, etc.

Jim
=========================================================