Pavia Experiment 14a Starting materials – Solubility, ionization, LD 50 Product properties – Solubility, boiling point, ionization Reactions – Overall & detailed steps End product – Structure, yield calculation Applications – fruit smell & flavor, Insect pheremones,

Starting Material, isopentyl alcohol also …. Iso-amyl alcohol, 3-methyl-1-butanol (better) BP=131.1 o C, density g/mL Slightly soluble in water = 28g/Liter – 1mL ≈ 0.028g dissolving in water LD 50 = 1300mg/kg (rat)... 91g for 154 lb person Bad smell

Starting Material, Acetic Acid (HOAc) BP=118 o C, density = 1.06 g/mL (pure=“glacial”) very soluble in water, polar – Ionization Ka=1.77*10 -5, not many ions to react – Add sulfuric acid as catalyst LD 50 = 3,310mg/kg (rat)…272g for 154 lb person

The Product, Isopentyl Acetate BP=142 o C, density g/mL, FW 130.2g/mol Slightly soluble in water at 0.3% LD 50 = 7422mg/kg (rat)...519g for 154 lb person

The Reaction overview

Reaction in detail, skeletons

Reaction Details, condensed formulas

Generic Reflux Setup water jacket condenses reactant & product vapors, returns to reaction vessel where higher temperature accelerates reaction BP= 131 o C(alc), 118 o C(acid), 142 o C product … condenser ≈ 20 o C

Isolate product from reactants Add NaHCO 3 solution to neutralize both acids NaHCO 3 + HOAc + H 2 SO 4  CO 2 (g) + Na 2 SO 4 +NaOAc Remove water layer & discard – Product is insoluble at 0.3% in water layer – Product floats (0.876 g/mL) on water layer Dry product – Pavia recommends Na 2 SO 4 This is STOPPING POINT for first lab day

Generic Distillation setup vapors from heated flask condense via cold water jacket thermometer at top indicates boiling point of product

Hickman Distillation for microscale small samples

Hickman Still Head inverted “pocket” catches small amounts of distillate

Can use round bottom flask & mantle

Analysis Yield – Measure mass of product, – compare to theoretical Boiling point – Value taken during distillation – 150 o C thermometer into Hickman head FTIR – Look for C-O-C & C=O absorption – Alcohol O-H should be absent

Reaction is 1:1 mole ratio

Yield Calculation Example (good format for lab notebook)

Chemical Analysis, Isopentyl Acetate Boiling Point – Literature value ≈ 142 o C FTIR, looking for C-H at ≈ 3,000 cm -1 C=O at ≈ 1,700 cm -1 C-O at ≈ 1,000 cm -1 FTIR, consumed alcohol should be missing C-OH at ≈ 3,300 cm -1

IR ABSORPTION RANGE The typical IR absorption range for covalent bonds is cm -1. The graph shows the regions of the spectrum where the following types of bonds normally absorb. For example a sharp band around cm -1 would indicate the possible presence of a C-N or a C-C triple bond.

Infrared Absorptions

INFORMATION OBTAINED FROM IR SPECTRA IR is most useful in providing information about the presence or absence of specific functional groups. IR can provide a molecular fingerprint that can be used when comparing samples. If two pure samples display the same IR spectrum it can be argued that they are the same compound. IR does not provide detailed information or proof of molecular formula or structure. It provides information on molecular fragments, specifically functional groups. Therefore it is very limited in scope, and must be used in conjunction with other techniques to provide a more complete picture of the molecular structure.

IR “Diagnostic Region”

IR “fingerprint” region

THE FINGERPRINT REGION Although the entire IR spectrum can be used as a fingerprint for the purposes of comparing molecules, the cm -1 range is called the fingerprint region. This is normally a complex area showing many bands, frequently overlapping each other. This complexity limits its use to that of a fingerprint, and should be ignored by beginners when analyzing the spectrum. As a student, you should focus your analysis on the rest of the spectrum, that is the region to the left of 1400 cm -1. Fingerprint region: complex and difficult to interpret reliably. Focus your analysis on this region. This is where most stretching frequencies appear.

IR vibrational motions Symmetric stretching Antisymmetric stretching Scissoring RockingWagging Twisting

GENERIC IR SPECTRUM OF AN ALCOHOL The most prominent band in alcohols is due to the O-H bond, and it appears as a strong, broad band covering the range of about cm -1. The sheer size and broad shape of the band dominate the IR spectrum and make it hard to miss.

Starting material IR, isopentyl alcohol C-H O-H C-O

Acetic Acid IR spectra

The –C–CO 2R stretch characteristic of an ester is visible in the isopentyl acetate spectrum in 1735–1745 cm -1 range. The –C–H stretches are visible just below 3000 cm -1, and –C–O and –CO 2 stretches appear as several peaks in the 1050–1300 cm -1 range.

Annotated IR absorption (linear scale)

Product Identification (log scale)

No IR hardware ? One of our 2 machines has no computer – Being replaced Second machine needs a replacement part – Part is on order If no IR, save your sample for testing later – Rest of report still due in 1 week – We’ll add the IR section when we can

Fun Part (Esters smells nice)

Fruit esters

Benzyl Acetate

Octyl Acetate Orange peel also contains Limonene, a natural insecticide

Isopentyl acetate is one example of a pheromone. It’s one of dozens of honey bee pheromones. It has been known since the 1960s that isopentyl acetate acts as an alarm pheromone for honey bees.

Heptyl butyrate Yellowjacket attractant pheromone, an ester

To the benches !