Midterm CHM 254

Question 1

Lab 2

Index of Hydrogen Deficency

Answer 1

(C+1)+(N-H-X)/2

Question 2

Lab 2

Explain a brief procedure of recrystallization

Answer 2

1. add product to water and boil until dissolve
2. hot filtration
3. cold filtration with vacuum
4. take mp of dried crystals

Question 3

Lab 2

eutectic point

Answer 3

the lowest possible melting point of a mixture

Question 4

Lab 2

melting point

Answer 4

where the liquid and vapor phases co exist

Question 5

lab 2

Percent recovery

Answer 5

m recovered/ mass initial

Question 6

Lab 2

percent recovery

w/o mass values

Answer 6

(1-Sc/Sh)*100

Question 7

Lab 2

Name some essential parts of choosing a solvent for recrystallization

Answer 7

1. the wanted product should be very soluble in hot solvent and very insoluble in cold solvent
2. bp of solvent should be lower than the mp of the wanted product

Question 8

Lab 3

Brief Procedure of extraction

Extraction of caffeine from tea

Answer 8

1. obtain tea+ CaCO3 + water and boil
2. cool boiled liquid and vacuum filter
3. set up sep. funnel and add filtered liquid + organic solvent + salt and shake to mix
4. Dry collected organic layer over MgSO4
5. gravity filter dried layer
6. rotovap
7. perform recrystallization w/ hot acetone + pet ether
8. take m.p of recrystallized caffeine

Question 9

lab 3

organic layer

Answer 9

the phase you want, compound + organic solvent

usually less dense than water

Question 10

lab 3

aq. phase

Answer 10

the water layer, water + any other impurities

usually more dense than organic solvent

Question 11

lab 3

Partition coefficient definition

Answer 11

equilibrium constant that determines the distribution of a solute between two solvents

Question 12

lab 3

Partition coefficent using solubilites

Answer 12

solubility of solute in organic solvent/solubility of solute in aq solvent

Question 13

lab 3

partition coefficient using concentration

Answer 13

the concentration of solute in organic layer/ concentration of solute in aq layer

Question 14

lab 3

partition coefficent >1

Answer 14

favors organic layer

Question 15

lab 3

partition coefficient < 1

Answer 15

favors aq layer

Question 16

lab 3

How do you determine the organic layer?

Answer 16

Density! Organic solvents are usually less dense than water and will be on top. Determine which solvent is less dense to know which layer is which.

Question 17

lab 3

why do you rotovap something?

Answer 17

to get rid of any additional organic solvent and obtain a solid product

Question 18

lab 3

How would you choose an organic solvent for extraction?

Answer 18

1. like dissolves like, chose a solvent that matches the polarity of the wanted compound
2. pick one with a large partition coefficent
3. Immiscible w/ water

Question 20

lab 4

Ir spectroscopy

Answer 20

the spectroscopy of functional groups

Question 21

lab 4

Dipole moment

Answer 21

separated positive and negative charges on a molecule due to differences in electronegativity

Question 22

lab 4

how does having a dipole moment affect IR spectra?

Answer 22

The greater the change in dipole moment during vibrational patterns the greater the absorption peak on the graph. So molecules with greater dipole moments will have greater absorptions

Question 23

lab 4

what does it mean to be IR active?

Answer 23

To be IR active there must be a change in your dipole moment during your stretching patterns. Diatomics and C-C or C=C bonds don’t have strong or any dipole moments to change.

Question 24

lab 4

In IR spectra a molecule can only move from the — energy state to the — — energy state

Answer 24

lowest, second lowest

n=0 to n=1

Question 25

# lab 4 Why does asymmetric stretching produce a more intense peak than symmetric stretching?

Answer 25

Asymmetric stretching creates a greater change in dipole moment than symmetric stretching.

Question 26

# lab 5 Procedure for TLC plates

Answer 26

1. obtain and cut spinach, add add methanol and crush 2. drain liquid, add hexanes and methanol, and crush again 3. drain into sep funnel, add water and shake 4. dry over sodium sulfate and gravity filter with cotton 5. rotovap 6. preapre TLC plates w/ determined number of applications

Question 27

# lab 5 what are the four solvents we prepared for TLC plates

Answer 27

hexanes: ethyl acetane (9:1) hexanes: ethyl acetane (7:3) hexanes ethyl acetane

Question 28

# lab 5 are hexanes polar or non polar

Answer 28

non polar

Question 29

# lab 5 stationary phase

Answer 29

the phase applied to the plates, has the binding sites

Question 29

# lab 5 is ethyl acetane polar or non polar

Answer 29

polar

Question 30

# lab 5 mobile phase

Answer 30

the solvent used, travels up the plate

Question 31

# lab 5 origin line

Answer 31

where the applications of solute are placed and where the solvent begins

Question 32

# lab 5 solvent front

Answer 32

where the solvent stops

Question 33

# lab 5 ratio to the front (rF)

Answer 33

the difference in the distance from the spot traveled to the solvent front

Question 34

# lab 5 Rf=

Answer 34

distance traveled by spot/ distance traveled by solvent = spot/solvent

Question 35

# lab 5 what is a binding site and why does it matter?

Answer 35

A binding site is where the mobile phase and the solute particles compete to bind to spots on the stationary phase

Question 36

# lab 5 what are the most commonly used stationary phases for TLC plates?

Answer 36

silica and alumina

Question 37

# lab 5 How would you chose a solvent for a TLC plate?

Answer 37

1. a polar solvent for polar mixture 2. a np solvent for a np mixture 3. if you have a mixure of polar and np then a mixture of the two will be needed for solvent

Question 38

# lab 5 would a polar or non polar solute travel further in a polar and non polar solvent mixture

Answer 38

non polar would likely travel further because it likely does not have electrophilic atoms that would form hydorgen bonds at the binding sites.

Question 39

# C-NMR Each peak corresponds to --- on a C-NMR

Answer 39

set of equivalent carbons

Question 40

# C-NMR Which side of a NMR is considered up field and downfield?

Answer 40

Left side is upfield, higher energy, deshielded Right side is downfield, lower energy and shielded

Question 41

# C-NMR Do pi bonds have higher or lower chemical shift on an NMR?

Answer 41

Higher chemical shift | important for carbonyl bonds and aromatics

Question 42

# C-NMR Do alkanes have higher and or lower shifts?

Answer 42

lower shifts | sp3 atoms have low shifts