ORGANIC CHEMISTRY RESPONSES

Your Name: Kose21nna

Alkynes have a very interesting structure as they consist of a sigma bond and two separate pi bonds to make up the overall triple bond characteristic of the alkyne functionality.

pKa = 25

homero

1.Consider the structures below and discuss the reason for the different pKa values being indicated for the C-H bond of a terminal alkyne vs. vinyl C-H (alkene) vs. alkyl C-H bond. From these values it is clear that the terminal alkyne is most acidic (lower pka). Your discussion needs to address the reason why rather than simply say "because the pka is lower". Why is the pka lower?

Terminal Alkynyl C-H

pka voalues tell

VS.

The lower the

The Carbon - carbon bon d

making

pka the more

pka = 44

Vinyl C-H

the acid strength, there fore it tells the level the solution is.

triple bond break double VS Or

Oue22

aidic

VS.

Aciditu decreases as electronegatwty de ereases

1

pKa = 50

Alkyl C-H

Alkynul C-H is more electr oneaatie

so it will take more energy to Single bonds.

by looking at hy br'idization

1t More audc

acdity Can e deter mined

S characBer is higher acisity

2. Terminal alkynes are a valuable carbon source due to their relatively more acidic C-H bond. Furthermore, the C-C triple bond of an alkyne can be further reacted upon to form other useful functional groups. Consider the transformation below and propose a synthesis for the product being indicated starting from the acetylene (otherwise known as ethyne). You may use any other reactions that have been covered in chapter 9 as well as those covered in previous chapters of the text. Show all reagents needed and draw the product expected after each step in your proposed synthesis. It may be helpful to draw out a retrosynthetic scheme first so that you have a workable plan for your synthesis. Make sure that the reactions used are consistent with the relative stereochemistry indicated in the product shown.

HH

HH

Co,H

Na NH2

HeC-C= CH

NH3

Hz, Pd

steps

HEc + N

Lindlars Cataly St

2

+ enantiomer

CH3Br

c HyCH2 CH3

3. Allenes have both similar and different structural properties compared to alkynes. Look up the three-dimensional orbital diagram for both an alkyne and an allene. Then, redraw each structure below showing the hybrid atomic orbitals clearly. Label the specific hybrid orbitals that overlap to make the sigma-bonds and also label cach of the p-orbitals clearly. It will be helpful to first know what the hybridization of cach carbon atom is when doing this so that you know what atomic orbitals (hybridized and unhybridized) are available to hold the bonding and non-bonding electrons. In your drawing. be sure to clearly illustrate the plane each of the pi-bonds are oriented about and explain why different plancs are required for each of the pi-bonds, respectively.

hybridization?

HC-CEC-H

(an alkyne example)

Sp

propyne

unhybridized p-orbitals ’2 sigrna bonds

Vs

They diCfer in their bond arrangenment (= vs =) Triples bonds are shorter and stronger Charder to break)

3

Sp2)

hybridization?

C=c=c

Sp)

H

1,2-propadiene (an allene example)

hydridI 2ation of Car bon both allene and alkyne

4. Interestingly, although the alkyne and allene look similar in terms of the orbitals, the protons atached to them have very different lH NMR chemical shifts. The allene proton chemical shift is approximately 4.5 ppm whereas the alkyne proton chemical shif is approximately 2.1 ppm. Propose a reason for why these two protons have drastically different lH NMR chemical shifts. (Hint: look up the concept of ring currents). Drawa picture that illustrates this explanation clearly.

2.1 ppm

H-cEc{) VS

VS,

These stay in a lower

taeld stren gth because of the pi-eleckrons n the = bond

-H

H,C=C

Therefore the shlf wil) be slowes

4.5 ppm (more downfield)

alkyne Electroneqatiuity effects chemical shi+

H

H

The bond would parallel with the magneic Field

alkene

4

double bonds are perpen dicolar to

the maone. +ie field

These protons have a much larger reld streng th , resulting higher Chemical Shif+