[CH 220C] Lab 6 - Reduction of 9-Fluorenone

Objective of the lab

Study the reduction of 9-fluorenone to 9-fluorenol using sodium borohydride

Data to collect

Percent yield, melting point, & IR

Overall reaction equation

9-fluorenone --[1) NaBH4 / MeOH]--> Alkoxide intermediate --[2) H2SO4]--> 9-Fluorenol

How can you tell the reaction occurred?

Loss of color; 9-fluorenone is bright yellow & 9-fluorenol is white

Expected IR results

No big carbon stretch peaks, weak OH peaks

Common methods of reduction

1) Catalytic hydrogenation: using H2 with metals, plus pressure & heat

2) Chemical reduction: using metal hydrides

3) Dissolving metal reduction: using finely iron or tine with HCl or alkaline earth metal in liquid ammonia

Catalytic hydrogenation

Reduction using H2 with a metal catalyst (i.e. Pt, Pd, Ni, or Rn), plus heat & pressure

When is catalytic hydrogenation commonly used to reduce carbonyl compounds?

Industrial processes

Chemical reduction

Reduction using metal hydride reducing agents (i.e. LiAlH4, NaBH4, etc.)

Traits of LiAlH4

- Very reactive & unselective

- Easily reduces aldehydes, ketones, & esters

- Must be used in anhydrous ethereal solvents (i.e. diethyl ether or THF)

Traits of NaBH4

- Less reactive & more selective

- Reduces aldehydes & ketones, but not esters

- Safer & easier to handle

- Can be used in alcohol & some aqueous solutions (i.e. MeOH in this experient)

Why is chemical reduction common done in aprotic solvents

Metal hydrides will produce H-, but can also produce H+ protons that for H2 very exothermically (big boom boom); solvents are often the largest proton source in a reaction, so it's best to use aprotic solvents

Order of metal hydride reactivity

NaH > LiH > CaH2 > LiAlH4 > NaBH4

Why are metal hydrides like NaH, LiH, & CaH2 not used in undergrad labs?

They are VERY reactive (metal electronegativities of ~1 & difference with hydrogen ~1.15) & mishandling them can burn buildings down

Why is NaBH4 less reactive than LiAlH4?

Since B is smaller, its valence electrons are closer to the nucleus & aren't given up more readily; makes B less electronegative

When is chemical reduction commonly used to reduce carbonyl compounds?

In research labs

Dissolving metal reduction

Reduction typically using finely divided iron or tin with HCl or an alkaline earth metal (i.e. Li, Na, K) in liquid ammonia

Selective reactions

Reactions yielding specific isomers of a desired product

Regioselectivity

Preferential formation of one constitutional isomer over another

Stereoselectivity

Preferential formation of one stereoisomer over another

Chemoselective reactions

Selective reactions that only change certain functional groups on a reactant

How can chemoselective reactions be achieved with catalytic hydrogenation?

Changing conditions like temperature & pressure (i.e. reduction of C=C, but not C=O, requires room temperature & 1+ atm)

How are carbonyls commonly reduced?

Carbonyls --> Alcohols

- Aldehydes --> 1* alcohols

- Ketones --> 2* alcohols

How are nitro groups commonly reduced?

Nitro groups (-NO2) --> Amino groups (-NH2)

Oxidation number

How many electrons an atom has gained, lost, or shared to become stable

Oxidation

Loss of electron density; increases oxidation number

- Often add Os and/or remove Hs

Reduction

Gain in electron density; decreases oxidation number

- Often add Hs and/or remove Os

How is reduction represented in chemical equations?

A "[H]" symbol above an arrow

What is important to remember about sodium borohydride (think moisture)?

Sodium borohydride is sensitive to moisture loses a hydride to water, meaning it can no longer be used as a hydride source; ensure it's not left uncapped

What is important to remember when adding sulfuric acid to the fluorenone-sodium borohydride solution?

Add the acid slowly in a dropwise fashion to prevent intense bubbling

Make sure the syringe is empty & wiped clean before returning to the basket

After adding methanol to the fluorenone-sodium-sulfuric acid solution, how should it be heated?

Start heating at 300 C, then once the vapor line on the condenser reaches 1/3 the way lower, lower heat to 150 C

What is important to remember when isolating your product, & what should you do if things go wrong?

Watch the receiving flask to make sure no product leaks out

If leakage occurs, pause filtration, transfer the filtered liquid onto a beaker, & filter again

What do you rinse/neutralize the product with during isolation, & how do you check pH?

Fill the Buchner funnel 3 times with DI water

Touch pH paper at the bottom tip of the funnel

How do you prevent methanol from leaking from the pipette?

Wet the pipette first with methanol to induce surface tension

What are the general rules for IR?

Keep it clean: only use IR with ungloved hands, & clean the plate with isopropyl alcohol before & after each use

Keep the area around the IR clean: don't leave chemicals hanging out around the IR

Follow directions: instructions are right above the computer, so don't f*ck up

Be patient: the IR's software is old, so give it time to cook

Look at the IR: make sure your IR charts make sense before you leave, & check with your TA if things look weird

How long should you turn the IR press?

Turn until the pressure indicator is 50% green

If the indicator turns red, unscrew the press so the anvil doesn't crack

What should you do if the IR says "the top plate can not be found?"

Ask your TA for help

Carcinogens & mutagens

Known carcinogens: sulfuric acid

Possible carcinogens:

9-fluorenone, 9-fluorenol

Known mutagens: none

Possible mutagens: methanol

Fire hazards

Negligible: sulfuric acid

Moderate: 9-fluorenenon, sodium borohydride, 9-fluorenol

Severe: methanol

Special fire protocols

Methanol: alcohol-resistant foam, CO2, dry chemical, or water

Sodium borohydride: dry chemical, sand, lime, soda ash

Special inhalation protocols

Sodium borohydride: seek fresh air, keep warm & at rest, get medical attention immediately

Sulfuric acid: seek fresh air, keep warm & at rest, get medical attention immediately

Special ingestion protocols:

Sodium borohydride: contact physician immediately, give water or milk, head under hips if vomiting occurs

Sulfuric acid: contact physician immediately, give water or milk, head under hips if vomiting occurs

9-fluorenol: don't induce vomiting, get medical attention if needed