Buffers and Extrations

Lab 1 Chem 241

Buffers 

Mixture of weak acid and weak conjugate base.

Ex: acetic acid/acetate, ammonium

equation: henderson hasselbalch

• If acid=base then it is log(1)=0.

Henderson Hasselbalch

Can be used when we know the concentration of both acid/basic components to calculate pH

Ex: acetic acid has a pKa of 4.5. if you mix together 1M acetic acid and 3M acetate, what is the pH of the buffer solution 

• pH= 4.5+log(3/1) = 5.0

Can also be used when given a defined ph what is the acid/base ratio?

Ex: An acetic acid/acetate buffer has a pH of 4.2, how much of each component?

• 4.2 = 4.5 + log(base/acid) 

• log(base/acid) = -0.3 and base/acid= 0.5

• 0.4 M acetate and 0.8 M acetic acid. 

grignard reagants 

• RMgX, strong nucleophile and bases and extremely basic can react w/ any proton source

• highly reactive compounds used to form new C-C compounds 

• carbonyl compounds reactive with aldeyhes/ketones to create alcohols

• Grignard reagents are destroyed by acids

grignard product mixture 

• mix depends on type of electrophile that it was reacted with and conditions 

• mixture used for: drying, evaporating, extraction

  • used to isolate desired product

Ex: Connecting Buffers and Grignard 

• How much base do we need to add to get all sodium benzoate that is water soluble?

  • Need buffer with 99% base and 1% acid, Henderson-Hasselbalch equation specifies we need 2 pH unit above pKa to obtain that goal. So if benzoic acid has pKa of 4.5 we would need pH to be 6.5 or higher.

  • acid neeeded to neutralize benzoic acid to make it perticipate would have to be a lower pH about 2.5.

NMR

• exceptions to the symmetry rule are cis/trans H’s are different from each other (diastereotopic)

  • Ex: alkenes and cycloalkanes and chiral systems

• cycloalkanes and symmetry:

• Ex: CH4 environments: 1 C and 1 H

TLC Plates

• lower Rf is more polar

• Higher Rf is less polar

diethyl ether in TLC plate

How do you know which compnent has a high or low boiling point in GC based off resutls?

• high retention = high b.p.

• low retention time = low b.p.

• Those with low retention time and low b.p. will come first in a simple/fractional distillation

How can you tell 2 components are present in the same amount in GC results?

• Same area and different retention time

S vs. R