It's such a difficult formation to play against: I played for Belgium against [Conte's] Italy at the Euros and, even though we saw plenty of the ball, it was so hard to break down that defensive block.
Compared to olefins, paraffins have a lower reactivity towards cracking due to energetically more difficult formation of carbenium ions.
But because they are deeply buried under difficult geological formations in dry regions where the soil is less porous, hydraulic fracturing or fracking will require twice as much water as is used in the U.S. and China already has a serious water shortage problem.
Several oil companies operating in Russia, including Ruspetro, have begun profitably extracting oil from shale rock and other difficult geological formations in Siberia, holding out hope that the type of advanced drilling techniques used in recent years in North America can be widely adopted there, too.
These challenges include more restrictive regulations, a negative public perception, and difficult geological formations.
Blind hole tapping in such materials is problematic due to difficult chip formation, chip transport, the tendency of cold-welding and build-up edges on the tool.
However, the large difference in melting points and vapor pressures between Mg and Ni make difficult the formation of high quality Mg2Ni [7].
Thus, another problem with the "prebiotic" RNA world is that the synthesis of purine nucleosides is somewhat robust, but nucleotide formation may be difficult, while nucleotide formation from pyrimidine nucleosides is robust, but nucleoside formation may be difficult.
When R is not sterically demanding, or when the second olefin is too substituted, the evolution of B syn into C is more difficult and the formation of D becomes competitive, thus leading to the formation of 3.[ 3d– f, 10] The same mechanistic manifold is operative for substrate 4 a, thus leading to the formation of 5 and 6.
Signing of non-permanent road surface conditions, such as ice, is difficult because hazard formation, location, and duration are unpredictable.
