Organic Chem Lab Exam 2-- Chemiluminescence and Luminol Synthesis

what is the purpose of this lab?

to synthesize luminol and investigate chemiluminescence reaction luminol is known for

what import lab process does this display?

multistep process of using a starting material, converting it to a product, and using the product of the first rxn as a starting material for a different reaction

what is luminol best known for?

use by crime scene investigators to detect blood

what type of solution will glow blue when sprayed in an area with a trace amt of blood?

solution of luminol, oxidizing agent (usually hydrogen peroxide), and base (usually hydroxide)

what is the glow of a rxn with luminol called?

chemiluminescence

what is the 'active ingredient' in blood that causes the glow?

iron in hemoglobin

how is chemiluminescence defined?

the release of light without heat due to a chemical reaction

how does chemiluminescence differ from fluorescence?`

the energy to produce light in a chemiluminescence reaction comes from the energy released by the breaking, formation, or restructuring of chemical bonds

light released from fluorescence reaction is caused by absorbance of light at a higher frequency then release at a lower frequency visible to the human eye

what is a popular example of chemiluminescence?

glow produced by light sticks

which starting material was reduced in the first portion of the experiment?

which portion of this molecule was reduced?

5-nitro-2,3-dihydrophthalazine-1,4-dione

the nitro functionality

what reagent was used to reduce 5-nitro-2,3-dihydrophthalazine-1,4-dione?

sodium hydrosulfite in a solution of 3 M sodium hydroxide in water

what type of pathway does the mechanism for reduction of 5-nitro-2,3-dihydrophthalazine-1,4-dione use?

mechanism uses a single electron pathway

what type of molecule results from the reduction of 5-nitro-2,3-dihydrophthalazine-1,4-dione and what is it used for?

luminol (an amine); used as starting material to investigate the chemiluminescent properties of luminol

which reagents are reacted in the 2nd portion of the experiment?

crude luminol reacted with 3 M sodium hydroxide, hydrogen peroxide, and potassium ferricyanide to cleave the dihydrophthalazine ring and form a phthalate salt along with the release of light

which reagents provide a molecular oxygen source for the rxn?

hydrogen peroxide and the iron catalyst (potassium ferricyanide)

what is the overall bond order of ground state molecular oxygen?

how does ground state molecular oxygen exist and what does this mean?

2

exists in the triplet state; meaning two unpaired electrons of the same spin

chemically, how is triplet molecular oxygen added to luminol?

by a complex mechanism catalyzed by a change in iron oxidation states

what results from when luminol is reacted with triplet molecular oxygen (via a complex mechanism catalyzed by change in iron oxidation states)?

what happens to this product?

a phthalate salt (5-aminophthalate salt)

the product molecule (5-aminophthalate salt) is forced into a T1 (triplet excited) state due to the addition of triplet molecular oxygen

what happens after the phthalate salt (product of luminol and the triplet molecular oxygen) is forced into the T1 state?

the 5-aminophthalate salt converts to the S1 excited state (excited spin-paired state) from the T1 excited state (excited spin-unpaired state) through intersystem crossing

why is blue light observed in the chemiluminescence reaction?

after the resulting phthalate salt converts from the T1 excited state (excited spin-paired state) to the S1 excited state (excited spin-unpaired state) through intersystem crossing, the molecule's electronic state relaxes to the S0 ground state and releases a photon which is seen as blue light

explain the energy release pathway causing chemiluminescence (p. 75)

first triplet excited energy level (T1) --> vibrational energy release --> intersystem crossing --> first singlet excited energy level (S1) --> vibrational energy release --> energy loss through fluorescence --> singlet ground state (S0)

what was the first step in luminol synthesis?

add 150 mg of 5-nitro-2,3-dihydrophthalazine-1,4-dione and add it to conical vial with spin vane, add 2 mL 3 M NaOH solution to conical vial and stir until solution is deep red/brown

what did we add to solution in luminol synthesis after reacting 5-nitro-2,3-dihydrophthalazine-1,4-dione with NaOH?

add 250 mg sodium hydrosulfite to solution and wash all solids from vial walls with no more than 1 mL of water

what did we do after we added sodium hydrosulfite to solution (rxn mix of 5-nitro-2,3-dihydrophthalazine-1,4-dione and NaOH)?

assembled reflux apparatus and refluxed solution for 5 min

what did we add to solution in the luminol synthesis portion after refluxing for 5 min?

cooled solution to room temp and added acetic acid and stirred for 5 min; then cool solution on ice for 10 min

what did we do after adding acetic acid in luminol synthesis portion?

cool solution on ice for 10 min; filter precipitate via vacuum filtration and rinse with ice cold water; continue to pull vacuum through aspirator to dry solid

what is the solid precipitate in this experiment?

what did we do with the solid precipitate?

luminol

record weight of resulting solid

which solutions did we make for the chemiluminescence portion of experiment?

1) stock solution A
2) solution A
3) stock solution B
4) solution B

how did we make stock solution A?

dissolve luminol product into 2 mL 3 M NaOH

how did we make solution A?

dilute 1 mL of stock solution A using 9 mL of water

how did we make stock solution B?

mix 5 mL of previously prepared potassium ferricyanide solution (3% by weight in water) and 5 mL hydrogen peroxide solution (3% by weight in water)

how did we make solution B?

dilute 5 mL of stock solution B with 15 mL water

describe how we conducted the chemiluminescence part of the experiment with the 4 solutions

dilute 3 mL solution A with 16 mL water using a beaker. in a dark room pour solution B into the beaker containing the diluted solution A. the solution should turn blue.

What is the compound being oxidized in the luminol synthesis reaction?

Sodium hydrosulfite is the reducing agent for 5-nitro-2,3-dihydrophthalazine-1,4-dione and thus is oxidized in the luminol synthesis reaction

What is the purpose of acetic acid in the luminol synthesis reaction? How would the amount of the luminol product obtained at the end of this reaction be affected if the number of moles of acetic acid added were less than the moles of NaOH added at the beginning of the experiment

Acetic acid is added to protonate the luminol and force precipitation from the aqueous layer. The purpose of adding NaOH to the solution at the beginning is to deprotonate the 5-nitro-2,3-dihydrophthalazine-1,4-dione and force it into the aqueous layer. If a smaller number of moles of NaOH were added, not as much 5-nitro-2,3-dihydrophthalazine-1,4-dione would enter the aqueous layer. Hence, not as much luminol product would exist to be protonated upon addition of acetic acid, and a smaller amount of luminol product would be obtained at the end of the reaction.

In the luminol synthesis reaction, why was COLD water used for rinsing in the filtration step? How would the efficiency of the filtration be affected if HOT water was used instead?

Cold water was used for rinsing in the filtration step to prevent the re-dissolving of luminol. The efficiency of filtration would be reduced if hot water were used because some of the luminol would likely re-dissolve into the aqueous layer.

When isolating the solid through precipitation and vacuum filtration, why was the solid not rinsed with a solvent like ethyl acetate or dichloromethane?

If an organic solvent like ethyl acetate or dichloromethane were used for the rinsing step instead of water, some of the luminol product would re-dissolve.

What is the difference because chemiluminescence and fluorescence? Where does the ENERGY for light emission come from in each case?

The energy used to produce light in chemiluminescence comes from the breaking, formation, or restructuring of chemical bonds. The energy used to produce light in fluorescence is caused by the absorbance of light at a higher frequency and then the release of light at a lower frequency visible to the human eye.

In the chemiluminescence reaction, a diene is generated on the luminol molecule. Why does this reaction not progress from a Diels-Alder reaction with molecular oxygen (O2) acting as the dienophile? How does ground-state molecular oxygen exist and does this affect the possibility of a Diels-Alder mechanism with O2?

Ground state molecular oxygen exists in the triplet state. A Diels-Alder reaction requires the formation of two sigma bonds. So, although a diene is generated on the luminol molecule, the reaction cannot progress via a Diels-Alder reaction because molecular oxygen cannot exist in a singlet state.

why is blood usually needed for fluorescence in the reactions with luminol?

the iron in blood could serve as the iron catalyst that provides the molecular oxygen needed for the rxn to occur

what are the 2 experimental parts of the overall lab?

1) reduce the nitro group on the starting material to an amine--this step synthesizes luminol
2) react luminol with base, peroxide, and iron

what happens when luminol is reacted with base, peroxide, and iron? (part 2 of overall experiment)

blue glow is emitted as a result of the electronic transitions of the triplet molecular oxygen

describe the make up of stock solution/solution A

this is the luminol solution; contains synthesized luminol and 3 M NaOH in water; dilute to make solution A

describe the make up of stock solution/solution B

oxygen source; 3% potassium ferricyanide solution in water and 3% peroxide in water; dilute to make solution B

triplet state oxygen is...

O2 ground state

the ground state of oxygen is the...

triplet state

what was the purpose of stock solution B?

to provide oxidizing agent and the catalyst for the chemiluminescence rxn

which reagent was the oxidizing agent in the CL part of the lab? which reagent was the catalyst?

hydrogen peroxide; potassium ferricyanide

what was the purpose of sodium hydroxide in the solutions of this experiment?

to create basic conditions for the reaction

what is the purpose of adding NaOH at the beginning of the experiment?

to deprotonate the starting material and force it into the aqueous layer