39Haan Avik those are not bulky groups. Ortho effect is not applicable for them....I read this on yahoo answers..the guy who answered was finishing his PhD in chemistry.
http://answers.yahoo.com/question/index?qid=20071012080118AABWL03
Zaahir si baat hai ki the group has to be bulky. It forces carboxyl group out of the plane.
Avik mere pyaare, dulaare, pinku, chintu, etcetc bhai....do NOT compare SNAr to EAS. Never. Comparing those two on the same basis is nothing but a crime which makes you liable to the CID.
What prevents SN in aryl substrates? Their delocalised electron density. What reduces this density? Deactivating groups. Will substitution become easier? YES.
All SNAr needs is a leaving group and something which reduces its electron density, making approach of nucleophile easier. Capiche?
And you were asking on chat about terminal alkynes giving Tollen's test. Actually baat yeh hai ki terminal alkyne mein removable hydrogen hota hai. This is replaced by metal atom in acid base type reaction. It gives a ppt of metal acetylide.
For eg, ethyne reacts with tollen's reagent to give AgC(triple)CAg, which has a yellow colour(not sure about the colour).