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For example, when you look at hex-1-en-5-yne, you'll find out that the terminal carbon with the triple bond has a reading of 69.7 ppm and the internal carbon with the triple bond has a reading of 86.1 ppm. This is significantly more downfield than the other sp3 carbons which have readings of 22.4 and 32.4 ppm, respectively. The other sp2 carbons have readings of 134.3 and 116.4 ppm, respectively. You can also see that the terminal carbons are more shielded, which makes me wonder why also they appear to be more electron rich?

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  • $\begingroup$ Try to see it in terms of electronegativity of the fragments considering directionality. That should help. I e a C hybridised sp is more electronegative as seen from the single bond as compared to the opposite triple bond region, which is packed by ekectrons. $\endgroup$ – Alchimista Jan 6 '18 at 14:37

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