ochem lab final Flashcards


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1

heat up sample in mel-temp too fast: what happens?

lag on thermocouple

inaccurate reading

2

mel temp capillary tube for inorganic compounds?

not usually, bc inorganic compounds often surpass mel temp MP range

3

name 2 immiscible pairs

water-hexane

pentane-diethyl ether

4

name 2 misciple pairs

water-ethanol

toluene-hexane

5

why is DMSO not routinely crystallized?

bc its boiling point is not 10 degrees C below its melting point

so it may "oil out" in solution

6

2 advantages of craig tube

able to be centrifuged

able to fit in a beaker water bath

7

disadvantages of craig tube

some mass lost in transfer

very small, not good if wanting to make a lot of crystals

8

purpose of activated charcoal

to remove colored impurities from compounds

if not added, black clumpy specks

9

seed crystals for recrystallization

provides a surface for re-crystallization to begin, esp. if not working

10

boiling stones

prevent rapid boiling and superheating

good for recrystallization and distillation

11

celite for recrystalization

keeps particles from getting thru filter paper during hot filtration

12

if used too much recrystallization solvent:

if tapping the flask, scratching with glass rod, and seeding didn't help

to fix, evaporate excess solvent, starting with about half of what needs to come off

13

stemless funnel purpose for recrystallization

less distance for product to fall down: prevents premature crystallization

14

boiling points are ___ at higher elevation

lower

15

three reasons to perform a vacuum distillation

compounds w/ boiling points > 200 decrees C. bc vacuum distillation allows it to be carried out at a lower/more convenient temp

when a compound decomposes at or near its boiling point

compounds unstable when exposed to atmosphere

16

when to use a simple distillation (2)

when separating a volatile substance from a non-volatile impurity, or when separating two volatile substances that have boiling points at least 30 degrees C apart

17

when to use fractional distillation

when two or more volatile substances need to be separated

18

RF formula

distance traveled by spot X/distance traveled by the solvent first

19

chromatography is based on the interaction of the components in a mixture with a ___ and ___ phase

stationary and a mobile phase

20

order of polarity for ascorbic acid, acetylsalicylic acid, acetaminophen

least to most polar: acetylsalicylic acid, acetaminophen, ascorbic acid

21

ascorbic acid polarity:

most polar bc polyhydroxylated nature

22

acetaminophen polarity due to:

second most polar due to OH and amide groups

23

if two compounds have identical boiling points, are they able to be separated by GC?

yes... factors such as polarity allow for dual separation

24

how to separate two compounds with identical retention times?

1. heat it to one's boiling points, and the one with the lower boiling point will travel first

2. make sure the proper column was chosen... this may take trial and error

25

longer carbon chain polarity

longer carbon chain=less polar

26

Higher retention time=

(gas chromatography)

the one with the most carbons, so one with the least carbons elutes first

27

After visualization, it is found that the spot did not move from the origin. What might you do to get the spot to move up the plate?

increase the SOLVENT polarity

28

a compound that has a high affinity for the stationary phase will come off the column BEFORE or AFTER a compound that has a low affinity for the stationary phase?

after

likes to be stationary so it waits

29

if you mix miscible solvents together, what happens?

a homogenous solution results

30

if two immiscible solvents are mixed:

less dense solvent is on top

31

this extraction takes advantage of:

This type of extraction takes advantage of the fact that most organic acids and bases are soluble in organic solvents while their conjugate acid or conjugate base ions are soluble in water.

so can get the acid and base separate at the end!

32

partition ratio

compound X in where its most soluble/compound X in what its less soluble

x partitions itself in btwn the two

33
  1. Sodium bicarbonate (NaHCO3) can be used as a base to extract carboxylic acids into the aqueous layer, but it cannot be used to extract most alcohols. Explain. Hint: the pKa of H2CO3 is approximately 6.
  • Because the pka of NaHCO3 is larger than H2CO3. But it is not larger than most alcohols!
34
  1. Diff btwn extraction and washing:
  • Extraction: selective transfer of a substance from one solvent to another solvent in which that substrate is more soluble
  • Washing is removing unwanted impurities from solvent that contains a compound of interest
35

methanol miscibility with water

they are miscible

36
  1. In this procedure your organic acid was isolated by precipitation and subsequent filtration. If the resulting organic acid had been soluble in the aqueous solution, how could it have been isolated?
  • Solvent such as ether could be added to extract the organic acid from the water layer
37
  1. You are given a two-phase system containing an aqueous layer and an organic layer, but you don’t know which is which, nor do you know the identity of the organic solvent. How could you determine which layer is the water layer?
  • Take a small drop of one of the layers. Add a drop of water in that layer and see if it dissolves. If so, that is the aqueous layer
38

dichloromethane and diethyl ether solubility

they are miscible, will form homogenous mixture

39

which is more dense, diethyl ether or water?

water

40

is NH2 or NH3+ more soluble in water?

NH3+, bc is conjugate acid of the base

remember, extraction takes advantage of the fact that most conjugate acids/bases are soluble in water, while acids/bases are soluble in ether

41

see prelab question 1 and post lab question 1 from the two step synthesis lab

...

42

what solvent needs to be used for the two step synthesis lab and why?

TEG, bc it has a very high boiling point (285 degrees C). ethanol, for example, is too low. need solution with a boiling point higher than 190 (experiment run at 190)

43

how does cyclohexane destroy excess bromine in the solution?

reacts to form 1,2-dibromocyclohexane

44

overall yield

multiply the percent yields of two steps

45

SN1 reactivity

allylic and benzyllic > tertiary > secondary >primary > aryl

46

n-butylbromide

the one with a primary bromide group on end

47

William ester synthesis is a type of

SN2

48

Why is it important to mix 2-naphthol and NaOh prior to adding 1-bromobutane

so that u can give OH a chance to deprotonate 2-naphthol to generate the desired nucleophile (the two rings with a (-) on it). otherwise, the OH may act as a nucleophile

49

acidity of 2-naphthol vs ethanol

2-naphthol is more negative bc its conjugate base has a negative charge that is stabilized by resonance

50

do you have to worry about NaOH deprotonating ethanol to any appreciable extent?

No bc the Keq for mixing those two is about 1, so not a major concern, as even if ethoxide does form, its bascicity is roughly the same as hydroxide and it can act as the base just like hydroxide to deprotonate 2-naphthol

51

fischer projections: review these!

...

52

symmetrical

IR inactive bc during the bond stretch there is no dipole moment

53

unsymmetrical

IR active bc there is a dipole moment during bond stretch

54

what is different between enantiomers?

optical activity

55

1-octENE vs 1-octanol ifr

1 octENE has a stretch for double bond that disappears and there is now a BROAD oh stretch

56

staggered, anti

biggest groups farthest away from each other

57

cyclopentane bond angles

cyclobutane

pentane: 109

butane: 90

58

if mirror images but two Hs

are IDENTICAL, not enantiomers, bc there was no chiral center switching bc of two identical groups

59

how to determine number of stereoisomers

2n where n=number of chiral centers

60

enantiomers means

ALL chiral centers switched!

diastereomers is just one switched

61

if asked to draw two diff models of 2 butENE, think...

cis and trans!

62

cis and trans are...

diastereomers!

63

boiling points and pressure relationship

directly proportional

64

what makes it a good choice for crystylization

3

boiling point ten degrees below melting point

miscible solvent pairs

crystallizes in the hot stage, not the cold stage

65

two techniques if not crystallizing

(3)

add a few pieces of pure compound crystals

scrape the side with a glass rod

celite