Some science questions

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1. Ice is less dense than water

Why this fact does result from strong hydrogen bonding between water molecule?

[from barron's ap bio]

2. How differenciate nucleophilic and base regarding SN2 E2? Does this something to do with metal?

[from organic chem review]

3. Cis-trans only applied to balanced structure such as double bonds or cyclo-even ring?

4. Regarding chirality, am I right in making two structures identical? I attached the picture..

Thank you!

 

[ushaseos]

1) Ice is less dense than water because the crystalline solid takes up more volume than the liquid. Water has its highest density at 4 Celsius. This has to do with strong H-bonding in liquid H2O. In general, you need to know ice is less dense because of H-Bonding and it is most dense at 4 Celsius. Don't worry too much about the specifics of Ice floating.

2) SN2 = weak base, strong nucleophile (things like I- or Br- or Cl-). Some strong bases like OH and OCH3 are ok too but things like (CH3)3C-O- (t-butyl oxide) are too strong.

A strong base will favor E2. Small, unhindered, strong bases like OH and OCH3 will favor the more thermodynamically stable inner double bond (example: CH3-CH2-CH=CH-CH3). T-butyl oxide will and other big bulky bases will prefer E2 where the double bond is on the outside (CH3-CH2-CH2-CH=CH2). Also heat is a great indicator that E2 is going to be the mechanism over SN2. There are 3 common ways of indicating heat: 1)Saying "Heat" 2) A capital Delta (triangle) 3) Showing a temperature that is warm/hot (like 50C or more)

3) Pretty sure its only Double bonds and ring structures

4) Yep, you got it!