what are structural isomers? draw two structural isomers of pentene
Answers
Answer:
Pentane has three structural isomers that are n-pentane, Iso-pentane (methyl butane) and neopentane (dimethylpropane).
Answer:
1
Lecture 6
Structural Isomers – Just how many structures can you make from a simple formula?
1. methane: CH4
C
H
H
H
H
CH4 C
H
H
H
H
condensed formula
2D formula 3D formula
methane
2. ethane: C2H6
Once the two carbons are connected, there are only six additional bonding sites and these are filled by the
six hydrogen atoms. Ethane is a saturated molecule. C2H6 is completely unambiguous.
C2H6
C
H
C
H
H
condensed formula 2D formula
3D formula
H
H
H
C C
H
H
H
H
H
H
You can twist about this single bond generating
different conformations. Build a model and try it.
condensed line formula
CH3CH3 ethane
The carbon-carbon single bond allows rotation of one group of three C-H sigma bonds past the other
group of three C-H sigma bonds. Using a molecular model of ethane, fix one carbon with one hand and
spin the other carbon with you other hand. The different arrangements of the atoms as they rotate past
one another are called conformations. Conformations are the result of differences in a structure from
rotation about single bonds. We will study conformations more in a later topic.
C C
R
R
R
H
H
H
The three front bonds can rotate around
the carbon-carbon single bond, like spokes
on a wheel or propeller blades on an airplane.
A straight-on view shows the three bonds as lines connecting to the front carbon atom,
written as a small dot. The rear carbon is usually drawn as a large circle and the three
bonds to that carbon are also drawn as lines, but only down to the circle. Rotation of
the front bonds is easily seen in two different structures. These are called Newman
projections and we will see more of them later.
H
H H
R
R R
= rear carbon atom
= front carbon atom
In this drawing the bonds
on the front carbon are
rotated by 60o
relative to
bonds on the back carbon.
H
R
R R
H
H
3. propane: C3H8
Once again, there is only one possible arrangement of the bonding atoms. The third carbon has to be
attached to either of the other two carbons forming a three carbon chain with eight additional bonding sites,
each bonded to a hydrogen atom. Propane is a saturated molecule. C3H8 is completely unambiguous.
2
Lecture 6
condensed line formula
CH3CH2CH3
C
C
C3H8
C
H
C
H
H
condensed formula 2D formula 3D formula
H
C
H
H
H
H
C
H H
H H
H H
H H
You can twist about either single
bond generating different
conformations. Build a model
and try it.
C
C
C
H H
Because the backbone of the
carbon skeleton (plus the two
hydrogen atoms at the end
positions) go up and down,
this shape is sometimes referred
to as a zig-zag shape. It can
also be called a wedge and dash
formula.
propane
Because single bonds allow rotation, there are a number of ways that propane can be drawn using slightly
different representations. If you have models, now is a time to use them. Keep them handy. You’ll want
to use them frequently.
C C C
H
H
H
H C C
C
H
H
H
H
H
H
H
H
H
H
H
H
H C C
H
C
H
H
H
H H
H
C C
H
H
C
H
H
H
H C C
C
H
H
H
H
H
H
H
H
H
H
This bond was
twisted up.
This bond was
twisted down.
This bond was
twisted up.
This bond was
twisted down.
The point you should get from this example is that three sequentially attached sp3
carbon atoms can be
drawn in a variety of ways, but are still the same structure. With additional carbon atoms (4, 5, 6…) the
possibilities increase, but the feature to focus on when deciding if two structures are identical or different
is the length of the carbon chain and the length and positions of its branches. Whether you have 3,4,5, or
more carbon atoms zig-zagged in any manner possible, you can always redraw it in a simpler straight
chain form. The straight chain representation is easier for you to work with, so choose it as your method
of drawing a 2D structure (even if it is changing its shape thousands of times per second). Drawing your
structures with a straight chain, as much as possible, will give you the best chance of not overlooking a
structure. We will use this approach in our subsequent examples.
4. butane and 2-methylpropane (isobutane): C4H10
We finally encounter an example where there is more than one possible carbon skeleton. First draw the
longest possible carbon chain, which would be...four carbons (you guessed it!). There are ten additional
bonding sites that are all saturated with hydrogen atoms. C4H10 is ambiguous.
C
C
C4H10
C
H
C
H
H
condensed formula
(ambiguous)
2D formula 3D formula
H
C
H
H
C
H
C
C H
H H
H H
H H H
H
H
H
H H
Rotation is possible about
any of the carbon-carbon
single bonds, generating
different conformations.
Build a model and try it. CH3CH2CH2CH3
condensed line formula
butane
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