With ethyl bromide, the reaction product is predominantly the substitution product. As steric hindrance around the electrophilic center increases, as with isobutyl bromide, substitution is disfavored and elimination is the predominant reaction. Other factors favoring elimination are the strength of the base. With the less basic benzoate substrate, isopropyl bromide reacts with 55% substitution. In general, gas phase reactions and solution phase reactions of this type follow the same trends, even though in the first, solvent effects are dropped.
A development attracting attention in 2008 concerns a S2 roundabout mechanism observed in a gas-phase reaction between chloride ions and methyl iodide with a special technique called "crossed molecular beam imaging". When the chloride ions have sufficient velocity, the energy of the resulting iodide ions after the collision is much lower than expected, and it is theorized that energy is lost as a result of a full roundabout of the methyl group around the iodine atom before the actual displacement takes place.
SN1 reaction
The S1 reaction is a substitution reaction in organic chemistry. "S" stands for nucleophilic substitution and the "1" represents the fact that the rate-determining step involves only one molecule (unimolecular). The reaction involves a carbocation intermediate. Some common S1 reactions are of secondary or tertiary alkyl halides under strongly basic conditions or, under strongly acidic conditions, with secondary or tertiary alcohols. With primary alkyl halides, the alternative S2 reaction occurs. Among inorganic chemists, the S1 reaction is often known as the "dissociative mechanism". Christopher Ingold et al. first proposed the reaction mechanism in 1940.
An example of a reaction taking place with an S1 reaction mechanism is the hydrolysis of tert-butyl bromide with water to form tert-butyl alcohol:
This S1 reaction takes place in three steps:
Because the first step is the bottleneck or "rate-determining step" chemists classify the entire reaction mechanism as S1. Only one molecule is needed for that step.
Sometimes a molecule can react using either an S1 or an S2 mechanism.
The S1 mechanism will win this competition when the central carbon atom is surrounded by bulky groups because such groups sterically hinder the S2 reaction. Additionally, bulky substituents on the central carbon increase the rate of carbocation formation because of the relief of steric strain that occurs. The resultant carbocation is also stabilized by both inductive stabilization and hyperconjugation from attached alkyl groups. The Hammond-Leffler postulate says that this too will increase the rate of carbocation formation. The S1 mechanism therefore dominates in reactions at tertiary alkyl centers and is further observed at secondary alkyl centers in the presence of weak nucleophiles.
An example of a reaction proceeding in a S1 way is the synthesis of "2,5-dichloro-2,5-dimethylhexane" from the corresponding diol with concentrated hydrochloric acid:
As the alpha and beta substitutions increase with respect to leaving groups, the reaction is diverted from S2 to S1.
The carbocation intermediate formed in the reaction's rate limiting step is an "sp" hybridized carbon with trigonal planar molecular geometry. This allows two different paths for the nucleophilic attack, one on either side of the planar molecule. If neither path is preferentially favored, these two paths will be used equally, yielding a racemic mix of enantiomers if the reaction takes place at a stereocenter. This is illustrated below in the S1 reaction of S-3-chloro-3-methylhexane with an iodide ion, which yields a racemic mixture of 3-iodo-3-methylhexane:
However, an excess of one stereoisomer can be observed, as the leaving group can remain close to the carbocation intermediate for a short time and block nucleophilic attack. This is very different than the S2 mechanism, which does not mix the stereochemistry of the product (a stereospecific mechanism). The S2 mechanism always inverts the molecule's stereochemistry.
Two common side reactions are elimination reactions and carbocation rearrangement. If the reaction is performed under warm or hot conditions (which favor an increase in entropy), E1 elimination is likely to predominate, leading to formation of an alkene. At lower temperatures, S1 and E1 reactions are competitive reactions. So, it becomes difficult to favor one over the other. Even if the reaction is performed cold, some alkene may be formed. If an attempt is made to perform an S1 reaction using a strongly basic nucleophile such as hydroxide or methoxide ion, the alkene will again be formed, this time via an E2 elimination. This will be especially true if the reaction is heated. Finally, if the carbocation intermediate can rearrange to a more stable carbocation, it will give a product derived from the more stable carbocation rather than the simple substitution product.
Solvents will change the reaction rate. Since the S1 reaction involves formation of an unstable carbocation intermediate in the rate-determining step, anything that can help this will speed up the reaction. The normal solvents of choice are both "polar" (to stabilize ionic intermediates in general) and "protic" (to solvate the leaving group in particular). Typical polar protic solvents include water and alcohols, which will also act as nucleophiles.
The Y scale correlates solvolysis reaction rates of any solvent (k) with that of a standard solvent (80% v/v ethanol/water) (k) through
formula_1
with "m" a reactant constant ("m" = 1 for tert-butyl chloride),
For example 100% ethanol gives Y = −2.3, 50% ethanol in water Y = +1.65 and 15% concentration Y = +3.2.
SC Austria Lustenau
SC Austria Lustenau is a football club from Lustenau, Vorarlberg. The club is playing in the Erste Liga, the second division in Austria.
The club was founded in June 1914 as football department of Turnerbund Lustenau and was named FA Turnerbund Lustenau. This name had the club till 1936. For not having a football assosiation in Vorarlberg the club only played friendly matches. During the time of First World War there were no matches. In 1920 the club started in a regular championship. The first success was the Vorarlberg championship in the 1929/30 season. They were qualified for the Austrian Amateur Championship and reached the final versus Kremser SC.
In 1936 the club left the Turnerbund Lustenau and became a club of its own. The were renamed to SC Austria Lustenau. In the following years they reached several titels. In 1994 they won the Regionalliga West and were promoted to the First League, the second division in Austrian football. From 1997 to 2000 they played in the Bundesliga, the top division of Austrias football.
The biggest success so far was the Cup final in 2010/11. Austria Lustenau was the first club from Vorarlberg which ever reached a Cup final. The match was lost against SV Ried.
"As of 12 August 2011."
Stuntman: Ignition
Stuntman: Ignition is a racing/action video game that was published by THQ and was developed by Paradigm Entertainment. It was released for the PlayStation 2, PlayStation 3 and the Xbox 360 systems. Stuntman: Ignition was the sequel to the 2002 video game, Stuntman. The game received positive reviews, GameSpot gave it a 7.5 out of 10 and IGN gave it a 7.1 out of 10.
Kazunori Yamauchi
Cradle of Humankind
The Cradle of Humankind is a World Heritage Site. It was first named by UNESCO in 1999. The site is about 50 kilometres northwest of Johannesburg, South Africa in the Gauteng province, and occupies .