Contradicting points in ExamKrackers?

[UIC]

Ok so one paragraph says this:

If a carbon chain contains a carbon-carbon double bond, it is an alkene. Alkenes have pi bonds. Pi bonds are less stable than sigma bonds; thus alkenes are more reactive than alkanes. When dealing with alkenes, remember that pi bonds are electron hungry. This explains why alkenes are more acidic than alkanes.

In another paragraph:

Electrophilic addition is an important reaction for alkenes. When you see an alkene on the MCAT, check for electrophilic additin. An electrophile is an electron-loving species, so it will have at least a partially positive charge, even if i tis only from a momentary dipole. The double bond of an alkene is an electron-rich environment and will attract electrophiles


Im kind of confused how can they be electron hungry and electron rich at the same time? Am I missing something here? Can someone please clarify this.

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[MediCynical]

Yeah, the wording is confusing when you put the paragraphs together like that.

In terms of being electron hungry, what they're saying is the double bond is going to be pulling electrons in the carbon chain towards it, so given H2C=CH-CH2-R, the electrons of the bolded -H's feel more of a pull from the double bond than they would from a single bond in an alkane.

Regarding electrophilic addition, there are 4 bonding e- between the carbons, so the e- localization between the carbons in double bond make it electron rich.

Hope that helps

 

[orgohacks]

Ok so one paragraph says this:

If a carbon chain contains a carbon-carbon double bond, it is an alkene. Alkenes have pi bonds. Pi bonds are less stable than sigma bonds; thus alkenes are more reactive than alkanes. When dealing with alkenes, remember that pi bonds are electron hungry. This explains why alkenes are more acidic than alkanes.

In another paragraph:

Electrophilic addition is an important reaction for alkenes. When you see an alkene on the MCAT, check for electrophilic additin. An electrophile is an electron-loving species, so it will have at least a partially positive charge, even if i tis only from a momentary dipole. The double bond of an alkene is an electron-rich environment and will attract electrophiles


Im kind of confused how can they be electron hungry and electron rich at the same time? Am I missing something here? Can someone please clarify this.

Wow. I can see how that is confusing.

Electron-withdrawing and electron-rich are used in two different senses here and that is causing problems.

For the first paragraph, think about the order of acidity - acetylene (sp) is more acidic than alkene (sp2) which is more acidic than alkane (sp). The extra s character helps to stabilize the negative charge. Hence, you can think of sp as being more "electronegative " than sp2 which is more electronegative than sp3. This is purely through the *sigma* bonds, however (the p orbitals are not involved). So yes, although I'd word it differently, the alkene is more "electron hungry" (i.e. electron withdrawing) than an alkane in this case.

The second paragraph is describing the behavior of the p orbitals (pi bond).
I think looking at alkenes as nucleophiles is helpful. In addition reactions, think of the alkene as the nucleophile and whatever you're adding to it (bromine, BH3, acid, etc.) as electron poor and thus an electrophile.

Almost all reactions in organic chem boil down to "nucleophile attacks electrophile", or "negative goes to positive", or "electron pair goes to hole", however you want to look at it.

So the first paragraph is talking about the acidity of the sp2 proton, whereas the second paragraph is talking about the reactivity of the pi bond.

(a more long-winded way of repeating exactly what MediCynical just said)

hope this helps - James