unit 3 ) sterochemistry

Question 1

what are isomers

Answer 1

isomers are molecules that have the same molecular formula but different structural formula

Question 2

what are

• structural isomers
• stereoisomers

Answer 2

• structural isomers :differ by the order in which the atoms in the molecule are joined together
• stereoisomers : atoms making up the isomer are in the same order, but still manage to have different three dimensional arrangement in space.

Question 3

the two forms of stereoisomers

Answer 3

geometric isomerism

optical isomerism

Question 4

when does geometric isomerism / cis trans isomerism occur?

which molecules does this often happen in?

Answer 4

• when there is restricted rotation somewhere in the molecule
• this often happens if there is a carbon-carbon double bond, or in rings joined by carbon atoms

Question 5

in unsaturated hydrocarbons a pi bond between neighbouring carbon atoms prevents ______ about the bond.

Answer 5

rotation

Question 6

diagram/example of geometric isomer

Answer 6

look in notes

-these two molecules are not the same despite having the same formula

Question 7

explain the molecule in terms of cis and trans isomers

Answer 7

cis isomer- has groups on same side of the bond (in the molecule the methyl groups are locked on the same side of the double bond.)
trans isomer - has groups on opposite sides of the double bond (in the molecule the two methyl groups are locked on opposite sides of the double bond.)

Question 8

diagram of geometric isomerism occurring when carbon atoms are joined in rings

Answer 8

look in notes

Question 9

ring; explain which is the cis isomer and which is the trans

Answer 9

• in the cis isomer the two groups point in the same direction relative to the plane of the ring
• in the trans isomer the two groups point in different directions relative to the plane of the ring

Question 10

to get geometric isomers you must have restricted rotation (often resulting from a carbon-carbon double bond or a ring) , what else must you have?

Answer 10

• the molecule must also have two different groups attached to each of the carbon atoms of the double bond
• it doesnt matter whether the left hand groups are the same as the right hand ones or not

Question 11

mp and bp of cis and trans isomers of 1,2 dichloroethane

- explain why the trans isomer has higher melting point

Answer 11

cis - mp -80, bp 60, trans mp –50, bp 48

• trans isomers pack better than cis isomers , straight shape of the trans isomer means it packs better than the cis ( with the U shape)
• the poorer packing in cis isomers means that the intermolecular forces are not as effective and so less energy is needed to melt the solid giving a lower melting point

Question 12

• explain why the cis isomer has the higher boiling point

Answer 12

in 1,2 dichloromethane, the cis isomer is a polar , and the trans is non polar

• ( both molecules contain polar chlorine-carbon bonds , however in the cis isomer they are on the same side of the molecule -one side of the molecule will therefore have a slight negative charge while the other side is slightly positive, making the cis isomer polar)
• there will be dipole dipole interactions as well as ldf forces so the cis isomer requires more energy to overcome these attractive forces , leading its boiling point to be higher than the trans isomer

Question 13

but 2 ene isomers

Answer 13

-

Question 14

what are optical isomers

Answer 14

• where each isomer is the mirror image of each other , this is known as optical isomers or enantiomers ,
• they are isomers which are non superimposable

Question 15

if a molecule conatins a tetrahedral carbon atom with ____ different groups attached to it , then it will be an optical isomer. These molecules can be described as ______

Answer 15

four

chiral

Question 16

meaning of superimposable and chiral

Answer 16

superimposable-can align with another shape/object exactly and show no perceptible difference
chiral
chiral - asymmetry in a way where its structure and mirror image are non superimposable

Question 17

optical isomers are identical in every physical property except….

Answer 17

except from their effect on plane polarised light

Question 18

what is plane polarised light, how is it brought about

Answer 18

• normally a beam of light consists of an infinite number of waves vibrating in all planes perpendicular to the direction in which the light travels
• if this beam of light is passed through a polariser (such as the polaroid film in polaroid sunglasses) all the vibrations are cut out except those in one plane
• the light emerging from the polariser is the plane polarised light

Question 19

what happens when plane polarised light is passed through a solution containing an optical isomer

Answer 19

the plane of the polarised light is rotated through a certain angle

Question 20

if this solution was replaced by an equimolar (same concentration) solution of its other optical isomer, what happens ?

Answer 20

it too rotates the plane of the polarised light through the exact same angle, but in the opposite direction
eg if one isomer of an optically active compound rotates the plane of polarised light by +50degrees in a clockwise direction, the other optical isomer will rotate it by -50degrees in an anticlockwise direction

Question 21

what is a racemic mixture and what’s its effect on plane polarised light
such a mixture is ____ _____

Answer 21

• racemic mixture- equimolar mixture of two optical isomers
• would have no effect on plane polarised light because the rotational effect of one would be cancelled out by the opposite rotational effect of the other
  such mixture is optically inactive