Study sets, textbooks, questions
Upgrade to remove ads
ochem lab final
Terms in this set (35)
What other method can be used to speed up recrystallization in case there are no crystals formed upon cooling the solution?
-scratch sides of flask with glass rod
-put flask over ice
-boil off some more solvent, there might be too much solvent
Why do we need to wash the organic layer with saturated NaHCO3 solution?
What gas is released in this step?
-neutralize any acid in the organic layer
-decreases the solubility of the organic layer into the aqueous layer
-Carbon dioxide gas
the first water wash of the organic layer during the workup does what?
-removes any ionic particles and any left over water soluble materials
what was the purpose of washing the organic layer with an aqueous solution of sodium bicarbonate?
-neutralizes the HCl
What other technique can be used to further purify the liquid product that was synthesizes in this experiment?
What is Chromic acid and how is it formed by the reaction of potassium dichromate and sulfuric acid?
-oxidizes aldehydes to carboxylic acids
-chromic acid is prepared by treatment of potassium dichromate with aqueous sulfuric acid
A positive chromic acid test indicates?
positive test: blue/green color change
aldehydes are present
A positive benedicts test indicates?
blue to red color change
aliphatic aldehyde present
A positive iodoform test indicates?
White/yellow precipitate formed
methyl ketone present
What is the white/yellow precipitate in the iodoform test?
When using a MEL-TEMP to obtain the melting point of a compound you should place the thermometer in the unit, turn the dial about 1/3 of the way for 5-10 minutes prior to placing your capillaries with sample inside?
Which of the following (select all answers that are correct) are true regarding recrystallization?
-Sample that are cooled too quickly will result in precipitation rather than recrystallization occurred.
-The process of recrystallization requires nucleation to occur which requires energy to occur, but at colder temperatures limits this process.
When extracting a compound from a solution it is better to extract several times with smaller volume portions than one extraction with a larger volume.
When would you choose to attempt a recrystallization?
When you're given an impure solid that's insoluble in all the right aspects.
Difference between Soluble and Insoluble/Immiscible Impurities effect on MP Range?
Only "soluble" impurities, which are incorporated into the crystal structure at the molecular level, can cause depression and broadening of MP range. Insoluble/immiscible impurities (dirt, charcoal) WON'T affect the MP range.
What happens if you heat the sample in the Mel-Temp too fast during MP Range Determination?
Heating too fast doesn't give sufficient time for the thermometer to equilibrate with the metal of the Mel-Temp. Thus, the thermometer temperature will always lag behind that of the Mel-Temp, and you'll observe an artificially LOW MP range.(In exp., you want a temp. rise of ~2°C/min to allow time for the thermometer to equilibrate with the temp. of the metal of the Mel-Temp).
For a particular mixture there are three compounds present, in which X&Y are impurities and Z is your desired compound. Based on the information below, how can you obtain (collect) Z as a pure compound from X&Y?DATA: X and Z are soluble in hot ethanol (the solvent) and Y is not soluble in hot ethanol. Y & Z are not soluble in cold ethanol, but X is soluble in cold ethanol. (Soluble means the compound dissolves in the solvent, whereas insoluble means it falls out of the solvent).
-dissolve the given mixture in cold ethanol
-y and z will remain a precipitate
- filter our precipitate
-X can be heated and the recrystallized to get it again
- dissolve Y and Z in hot ethanol
- Y will not dissolve and a precipitate can be filtered out
- Z will form a solid once the hot ethanol is cooled
Using Table 1-2 on page 61 in your lab manual. If you are using benzene as a solvent, but need to use a second one along with it. What solvent from this table should NOT be used with benzene and give a brief statement as to why not.
Water should not be used with benzene. In the table water is the only solvent that is marked with I which means that benzene and water are not miscible with each other. Benzene and water will not form a homogenous mixture together. Benzene has no dipole movement and is non polar while water is a very polar molecule.
1. After your experiment you obtain a melting point of 71-77 degrees Celsius. What does this say about your recrystallization you preformed. Be details, yet brief, in your answer. (Don't just say it gives you the compound. Is it pure, is it impure, why too either or both?)
This wide range of a melting point indicates that the compound contains impurities. If the compound was pure the melting point would have a very small range
1. After filtering your recrystallized compound you wash it with COLD solvent. Why did you not wash it with HOT solvent?
Hot solvent would dissolve the crystals again while cold solvent insures that the crystals will stay in their same form. Hot solvent could potentially cause the loss of all of our crystals.
What are the two types of solvents you should choose in liquid/liquid extraction?
organic and aqueous
What are the criteria for choosing an organic solvent in liquid/liquid extraction?
inert, immiscible with water, good solvent, not too expensive, not too toxic, volatile
Why is the SLOW acidification of bicarbonate solution advised?
If too much acid is added at once it will form CO2 and water. So be careful not to do this or you will lose the contents of your beaker
Why is it advised to cool in ice during neutralization?
Because the reaction of acid and base releases heat.
How do drying agents work?
They are in their anhydrous form and react with polar reactants
When would someone use fractional distillation instead of simple distillation?
Separating complex mixtures of liquids with smaller boiling point separations
When would someone use simple distillation instead of fractional distillation?
Separating relatively pure liquids with large boiling differences or liquids with solid impurities
What are the two reasons to take melting points?
Determination of purity, identification of unknowns
Why is the melting point of an impure compound lower than the melting point of a pure compound?
If there are defects in the crystal structure caused by the presence of impurities, it takes less energy to break the crystal structure. Resulting in a lower melting point.
You want to isolate oxalic acid from a mixture, its solubility in various solvents is: 9.5g/100g water; 23.7g/100g ethanol; and 16.9g/100g in diethyl ether. You want to extract into one of the organic solvents to water. Which is the correct choice?
Diethyl ether. Even though the compound is more soluble in ethanol, ethanol is miscible with water and for extraction the two layers need to be immiscible.
How did the SN1 reaction occur?
2-methyl-2-butanol is reacted with HCl (place in separatory funnel and then shake) , a carbocation is formed, and the Cl will combine to form 2-chloro-2methylbutane
Experimental Components and their Roles in SN1-
baking soda and a weak base; neutralizes excess acid present
Experimental Components and their Roles in SN1- NaCl
salt water aka brine; decreases the formation of emulsions and promotes the separation of organic material from the water layer
Experimental Components and their Roles in SN1- calcium chloride
dehydrating (drying) agent; absorbs water from materials with which it comes in contact (used to dry liquids - remove H2O)
Other sets by this creator
Recommended textbook solutions
Janice Gorzynski Smith
K. Peter C. Vollhardt, Neil E. Schore
General, Organic, and Biochemistry
Danae R. Quirk Dorr, Joseph Topping, Katherine Denniston, Robert Caret
Other Quizlet sets
Pathology Exam 3 - Autoimmune Diseases
Instruments ch. 13
Con Law - Quiz 2