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Revising organic chemistry
Doc Brown's
Summary of organic reaction mechanisms
Part Ia Alkanes
Revision
notes
include full diagrams and explanation of the mechanisms and the 'molecular' equation and reaction conditions
and other con-current reaction pathways and products are also explained.
Revision notes for GCE Advanced
Subsidiary Level AS Advanced Level A2 IB
Revise AQA GCE Chemistry OCR GCE Chemistry Edexcel GCE Chemistry Salters
Chemistry CIE Chemistry revising courses for pre-university students
(equal to US grade 11 and grade 12 and Honours/honors level courses)
Part 1a
ALKANES
-
Alkanes
are not very reactive molecules. Most
reactions require some energy input to initiate a reaction e.g.
high temperature and catalyst for cracking, uv light for chlorination or a spark
to ignite them (initiating free radical reactions).
-
A combination
of two
main reasons account for this lack of reactivity compared to most
other homologous groups of organic molecules.
-
Bond
Strength:
-
The
single covalent
C-C (bond enthalpy 348 kJ mol-1) and C-H
(bond enthalpy 412 kJ mol-1) bonds are very strong so bond fission does not
readily happen. The carbon atom radius is small, giving a short and
strong bond with other small atoms. Therefore the reactions
will tend to have high activation energies resulting
in slow/no reaction.
-
Nature of
bonding:
-
Carbon
and hydrogen have similar electronegativities, so there is
no polar bond giving a slightly positive carbon (Cδ+) which
can be attacked by electron pair donating nucleophiles.
[e.g. see
halogenoalkanes (δ+C-Clδ-)
or aldehydes/ketones
(δ+C=Oδ-)]
-
All the
C-C and C-H bonds are single covalent and no region of
particularly high electron density
susceptible to attack by electron pair accepting
electrophiles. [e.g. like a double C=C bond see
alkenes which are highly reactive despite the fact the
C=C double bond has]
The free radical chlorination/bromination of alkanes
-
The basic
reaction is: R3C-H + Cl2
==heat/uv==> R3C-Cl + HCl
[mechanism
6]
-
The reaction is
initiated by higher temperatures e.g. 250-400oC or uv
light at room temperature.
-
If other hydrogen
atoms are available on the original hydrocarbon then polysubstituted
chloroalkanes will be formed

mechanism 6 -
free radical chlorination of an alkane,
when all R's are H, CR3H = methane
-
Step
(1)
is the initiation
step when the chlorine molecule is split into two chlorine atoms/radicals
by homolytic bond fission by the impact-absorption of the
ultraviolet photon. Its quantum of energy, E=hv, must
be great enough to break the Cl-Cl bond.
-
Homolytic
bond fission
means the original
pair of (Cl-Cl) bonding electrons is split between the
two radicals formed.
-
Step (1)
illustrates how to use
half-arrows to show a homolytic bond fission step
-
The red dots represent
the unpaired electron on the
free radical and the half-arrows show the individual electron
'shifts'.
-
The
breaking of the Cl-Cl bond in the chlorine molecules begins the
reaction because it is the weakest of the bonds of any reactant
molecule involved. Bond enthalpies/kJmol-1:
Cl-Cl = 242, C-C = 348, C-H = 412, and even the new bond
formed, C-Cl, is 338.
-
Free
radicals are highly reactive species with an unpaired electron
and tend to form a new bond as soon as is
possible by e.g. in this case by ...
-
Steps
(2) and (3)
are chain propagation steps, because as well as producing one
of the reaction products, a new free radical is also produced to
continue the reaction, which is why such reactions are sometimes
referred to as 'chain reactions'.
-
Steps
(4) to (6)
are three possible chain
termination steps which remove the highly reactive free radicals
as two unpaired electrons form a new bond, in this case single C-C
covalent bonds.
-
FURTHER
COMMENTS
-
The
mechanism for bromination is similar.
-
When the
alkane is methane, traces of ethane are found in the final
mixture of products. This provides evidence for a mechanism
involving a methyl radical.
The
free radical thermal cracking
of alkanes
-
e.g.
CH3CH2CH3
==> CH4,
CH2=CH2, CH3CH3,
CH2=CHCH3, H2
[mechanism
31]
-
When alkane
hydrocarbons are heated to a high temperature (450-900oC,
with/without superheated steam) they are thermally decomposed
or
'cracked' to form mainly alkanes of lower C number, alkenes of
equal or smaller C number and hydrogen.
mechanism
31 - free radical thermal cracking of alkanes
-
When the
temperature is high enough, the kinetic energy of the particles is
sufficient to cause bond fission on collision, and this initiates a
free radical chain reaction.
-
Step
(1)
is the
initiation step when a C-C bond in an alkane molecule is split
into two alkyl free radicals
by homolytic bond fission. This means the original C-C bonding
electron pair is split between the two alkyl radicals formed.
-
The weakest
bonds will break first in the initiation step, so the C-C
bond (bond enthalpy 348 kJmol-1) will tend to break
first as the C-H bond is stronger (bond enthalpy is 412 kJmol-1).
-
The
red dots represent the
unpaired electron on the free radical.
-
Free
radicals are highly reactive species with an unpaired electron
and tend to form a new bond as soon as is possible, in this
case by ...
-
Steps
(2) to (6)
are chain propagation steps,
because as well as a product, a free radical is also produced to
continue the chain reaction
and lead to, in this case, a variety of other products.
-
Steps
(7) and (8)
are two possible chain
termination steps which remove the highly reactive alkyl free
radicals. The unpaired electrons from the two radicals 'pair up' to
form a new bond.
-
FURTHER
COMMENTS
An
ionic mechanism for catalytic cracking
-
(a) Catalytic
cracking
occurs at lower temperatures
over a ceramic/zeolite catalytic material based on silica and/or
aluminium oxide (I've called it Z in the mechanisms). The mechanism
is believed to be ionic and some examples of reaction steps are
given below. Z represents a matrix element of the
alumino-silicate catalyst and can act in a variety ways e.g. Lewis
acid or base, Bronsted-Lowry acid or base. R1,
R2, R3 = alkyl
groups.
-
Step 1
Initiation: This
can occur via an alkane or an alkene previously formed from an
alkane.
-
Step 2
Propagation: This
example shows chain scission forming a 'smaller' alkene and a
'smaller' carbocation to continue the chain.
-
Step 3
Termination: A
lower alkene or alkane is formed.
| (indonesia)
Merevisi kimia organik Doc Brown Ringkasan mekanisme reaksi
organik Part Ia Alkanes Bagian IA Alkana Catatan Revisi
termasuk diagram penuh dan penjelasan tentang mekanisme dan
'molekul' persamaan dan kondisi reaksi dan reaksi lainnya
saat ini jalur-con dan produk juga dijelaskan. * (spanish)
La revisión de la química orgánica de Doc Brown Resumen de
los mecanismos de reacción orgánica Part Ia Alkanes Parte I
bis alcanos Notas de revisión incluyen diagramas completa y
explicación de los mecanismos moleculares y la "ecuación y
condiciones de reacción y otra corriente-con las vías de
reacción y los productos también se explican. * (chinese)
修改有機化學 督布朗的 有機反應機理綜述 部分前胡甲烷烴
修訂充分注意到包括圖表和解釋的機制和'分子'方程和反應條件和其他CON組電流反應途徑和產品也解釋。 前胡甲 烷烴 部分
-介紹( 這個頁)自由基氯化/溴化給halogenoalkanes( 鹵代烷,烷基鹵化物
自由基熱裂解給予較短的烷烴和烯烴 離子催化裂解烷烴和烯烴給予較短 *
(chinese-F) 修改有機化學 督布朗的 有機反應機理綜述 部分前胡甲烷烴
修訂充分注意到包括圖表和解釋的機制和'分子'方程和反應條件和其他CON組電流反應途徑和產品也解釋。機制的
介紹和使用的術語在有機化學 機制的指標 前胡甲 烷烴 部分 -介紹( 這個頁)
自由基氯化/溴化給halogenoalkanes( 鹵代烷,烷基鹵化物 )自由基熱裂解給予較短的烷烴和烯烴
離子催化裂解烷烴和烯烴給予較短 * |
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