Experiment 7: Dehydration of 2-methylcyclohexanol, tests for unsaturation, and gas chromatogrpahy

Reaction coordinate and Energy levels corresponding to the products: The major product in the above rxn coordinate is the more thermodynamically stable ____________________. The transition state leading to this product is lower in energy than the transition state leading to the ____________ product making it kinetically favorable. The relative stability of the two transition states is the same as that for the two products because structures of each resemble each other

1-methylcyclohexene; 3-methylcyclohexene

Procedure: PART 2- Tests for Unsaturation

1. Add 2-3 drops of starting material to two clean test tubes 2. Add 2-3 drops of dried product to additional test tubes 3. add bromine solution to both test tubes. If alkene is present, the red color of the bromine solution will become discharged upon addition 4. add potassium permanganate to both test tubes. if alkene is present a brown solid will form upon addition

Gas chromatography process: 1. Inject sample into GC instrument 2. Sample is instantly volatized into gas phase 3. ___________ gas (usually N2) moves gaseous sample through instrument to column 4. column separates components based upon bp 5. sample is detected by differing conductance of carrier gas vs. carrier gas+sample 6. Printer records resulting chromatogram

Carrier;

Qualitative unsaturation tests: bromination and potassium permanganate--- indicate presence of unsaturation but ___________ indicate regiochemistry of reaction

DO NOT

The Height equivalent to a theoretical plate (_________-) is the length of a column in which one simple equilibraton step occurs. The length of the column divided by the HETP is the effective number of partitions a compound undergoes in its passage through the column. The number of theoretical plates (n) is is given by formula: n=16(d/w)^2 d=distance in units of time between point of injection and point of elution; w= width in units of time of the peak at its base

HETP

Procedure: PART 3: Gas chromatography

Injuect 0.5 microL of dried product into gas chromatograph. Allow materials to elute through the column and separation to occur 2. record chart speed and column temp during separation 3. once separatin is complete, identify peaks in chromatogram by comparing bp of different components 4. determine retention time for individual peaks--- measure distance along x-axis between masimum of each alkene peak and the initial injection point, using chart speed convert distance measurements to time measurements 5. determine # of theoretical peaks (N) for each peak 6. Determine percent composition of the alkenes in the product- calculate area of each alkene peak and calculate percent area in relation to total alkene peaks

Why not use sulfuric acid instead of phosphoric acid? Sulfuric acid is a much _______ acid than phosphoric acid and easily protonates olefins to form carbocations which then react with water to form alcohols. This is reverse of desired transformation.

stronger

Tests for unsaturation: Permanganate KMnO4 will not react with alkanes. KMnO4 will cause _________ addition of two OH groups across the alkene forming the product and MnO2 as a brown precipitate

syn

Tests for Unsaturatoin: Bromination Br2 does not react with alkanes--- with alkenes Br2 is added in _________ formation to both ends of alkene causing the orange color to disappear

anti

A thermal conductivity devise is used for compound detection--- a wire heated by a constant voltage is placed in stream of helium carrier gas at the exit port of the chromatograph. The helium conducts heat away from the wire and cools it to a constant temperature. Helium conducts heat away from the wire much more efficiently than any organic compound. Thus when a sample elutes from the column, the wire heats up, its resistance decreases, and a current ____________.

flows

The acid-catalyzed dehydration of 2-methylcyclohexanol can occur in two directions to give either 1-methylcyclohexene or 3-methylcyclohexene. In this experiment, the course of rxn was determined by analyzing rxn mixture by ________ chromatography

gas

Best gas for gas chromatography?

helium

In theory, all chemical reactions are reversible--- if given enough time and enery, the reaction will occur and revert until an equilibrium concentration of A and B results- that position of equilibrium depends on the relative ergies of A and B-- if given less time and energy, the reverse reaction may not occur due to insufficient amount of heat required to reach ______ state

transition

Procedure: PART ONE- dehydration of 2-methylcyclohexanol

1. obtain microdistillation flask from stock room and assemblbe apparatus--- secure flask, slide thermometer into teflon adapter making sure thermometer is BELOW side arm of flask, secure collection flask, add sand to heating mantle until flask bulb is mostly covered, make sure apparatus is perfectly straight and sealed, check apparatus with TA, once approved add boiling stone and 2 mL of 2-methylcylohexanol and 2 mL of phosphoric acid 2. heat sample gently keeping vapor temp BELOW 96 degrees C(If you go above 96 the alkene will decompose and you can get some of the starting product in your distillate flask). Distill contents until 0.5-0.7 mL of liquid is collected 3. Dry product with calcium chloride over period of 10 minutes

quantitites injected on the column during gas chromatography are on the order of _________ to ___________moles

10^-5 to 10^-6

Thermodynamic control--> the lower energy ______ will preferentially form. This is because reverse rxn can occur and product equilibrium is reached- long rxn time, high temp, no removal of product. Our goal: form thermodynamic product

PRODUCT

What would happen if acid catalyst were not employed? Role of catalyst is to weaken the C-OH bond and therefore lower the _____________ required to produce the carbocation intermediate

activation energy

Reaction mechanism: OH group on 2-methylcyclohexanol attacks the hydrogen of the OH group on hydrogen phosphate forming water on cyclohexane. Water leaves as a leaving group forming a ________________. Water attacks the hydrogen bonded to the carbon wit h the methyl group forming a double bond and 1-methylcyclohexene.

carbocation

Steam Distillation: The product olefins are removed form the rxn mixture by distillation at a temperature that is below that of the starting alcohol, product olefin , and water. for immiscible liquids: Ptotal= vapor pressure of pure component A and vapor pressure of pure component B A mixture of two __________ liquids will boil at a ___________________ temperature than the boiling points of either of the two components. The composition of the vapor above the solution is determined only by the ratio of the vapor pressures of the two substances that co-distill. VP of pure A/VP of pure B = composition of vapor = Moles A in vapor/ Moles B in vapor Hence the vapor composition remains CONSTANT during distillation of immiscible liquids until one of the components is removed. Temperature never exceeds that of boiling water in steam distillation

immiscible ; lower

Gas Chromatography: Separation based on distribution of two or more compounds between two phases: Stationary (_________) phase: usually wax-like material consisting of long-chain hydrocarbons, silicone oil, or polymeric ethers coated on finely divided SiO2 Moving(______) phase: usually helium or nitrogen. The retention time is the time required for a compound to pass through the column.

liquid; gas

Kinetic control---> the product with the _________ energy TRANSITION STATE will preferentially form. This is because the revrese reaction can NOT occur and product equilibrium cannot be reached-- short rxn time, low temp, quick removal of product

lower

Factors affecting separation: 1. Nature of absorbent and its concentration on solid support 2. Vapor pressure (volatility) of compounds being separated. Compounds of ________ boiling point usually travel through the column faster than those of _______ bp 3.the higher the column temperature, the longer the time spent in the gas phase. High temp means short retention time and poor separation of components 4.Fast flow rate of carrier gas will prevent equilibration of the sample with the liquid phase. Short retention times and poor resolution 5. The longer the column length, the greater the number of theoretical plates. More theoretical plates means longer retention times and higher resolution

lower;higher

Quantitative analysis is possible by examining the area of the peaks on the chromatograph. The area under a gas chromatographic peak is proportional to the __________ of compound eluted. The area of a GC peak is determined by triangulation using formula: Area = H(W/2) H= height of peak; W/2= width of peak at half of its height the time required for helium to pass through the column is given by the position of the air peak.

moles

Hyperconjugation stabilizes pi bonds (pi--> sigma*): As a result of inductive effects and hyperconjugation, the order of stability for alkene is: unsubstituted< _______<________<Tri

mono; di

Why not use HCl instead of phosphoric acid? The chloride anion is a strong enough ______________ to react with the carbocation to produce an alkyl chloride

nuclophile

1. Hyperconjugation stabilizes carbocations (sigma--->p): as a result of inductive effects and hyperconjugation, the order of stability for a carbocation is methyl<____________<__________< tertiary

primary; secondary

the reaction is driven to completion by removing one or more of the _________ as the reaction proceeds

products

Why doesn't the dihydrogen phosphate anion react with the carbocation to produce alkyl dihydrogen phosphates? The lone pair of electrons on the phosphate anion are spread out (delocalized) over several atoms due to ______________ and therefore the dihydrogen phosphate is a weaker nuclophile than the chloride anion

resonance

Carbocation rearrangement/stability Hyperconjugation: electrons are able to go into empty p-orbital of the cation sp2 orbitals in alkene have more __character which makes them more positive Aromatics are stable because form one huge p-orbital that they all share which allows for hyperconjugation

s