Types Of Substitution Reactions Biology Essay

Types Of Substitution Reactions Biology Essay In a substitution reaction, a functional group in a particular chemical compound is replaced by another group .In organic chemistry, the electrophilic and nucleophilic substitution reactions are of main importance. Organic substitution reactions are classified into depending on whether the reagent that brings about the substitution is considered an electrophile or a nucleophile, whether a reactive intermediate involved in the reaction is a carbocation, a carbanion or a free radical or whether the substrate is aliphatic or aromatic. A reaction can be made faster or slower by taking into consideration the temperature and the solvent we are using.A good example of a substitution reaction is the photochemical chlorination of methane forming methyl chloride. Nucleophilic substitution What is a nucleophile Nucleophilic substitution happens when the reagent is a nucleophile, which means the attacking species is a nucleus loving species .it is itself negatively charged or has a lone pair. Such species get attracted to positive or electron deficient carbon centres.. A nucleophile reacts with an aliphatic substrate in a nucleophilic aliphatic substitution reaction. These substitutions can be of two dofferent mechanisms: unimolecular nucleophilic substitution (SN1) and bimolecular nucleophilic substitution (SN2). The SN1 mechanism has two steps. In the first step, the leaving group leaves, forming a carbocation. In the second step, the nucleophilic species attacks the carbocation and forms a sigma bond. This mechanism can result in either inversion or retention of configuration. An SN2 reaction has just one step. The attack of the reagent and the expulsion of the leaving group occurs simultaneously. This mechanism always results in inversion of configuration. When the substrate is an aromatic compound the reaction type is nucleophilic aromatic substitution. Electrophilic substitutions What is an electrophile An electrophile is a electron loving species, it itself is positively charged and wants to stabilize itself by making a sigma bond with the electron rich carbon centre. Electrophiles are involved in electrophilic substitution reactions and particularly in electrophilic aromatic substitutions. SN1 REACTION The SN1 reaction is a substitution reaction. SN means nucleophilic substitution and the 1 represents the fact that the rate-determining step is unimolecular . Carbocation intermediate is formed in this reaction .It is seen that tertiary carbocations are very stable due to + I effect and thus go for SN1 Reaction. With primary alkyl halides, the alternative SN2 reaction occurs. As primary Alkyl Halides mostly Form primary carbocation which is very unstable and thus have to go through SN2 Substitution reaction. Mechanism An example of a reaction taking place with an SN1 reaction mechanism is the hydrolysis of tert-butyl bromide with water forming tert-butyl alcohol: This SN1 reaction takes place in three steps: Formation of a tert-butyl carbocation by separation of a leaving group (a bromide anion) from the carbon atom: this step is slow and reversible. Nucleophilic attack: the carbocation reacts with the nucleophile. If the nucleophile is a neutral molecule (i.e. a solvent) a third step is required to complete the reaction. When the solvent is water, the intermediate is an oxonium ion. This reaction step is fast. Deprotonation: Removal of a proton on the protonated nucleophile by water acting as a base forming the alcohol and a hydronium ion. This reaction step is fast. WHY THIS REACTION OCCURS Bulky atoms(methyl, ethyl) surrounding the carbon atoms mostly allow SNI reaction. As the bulky alkyl halides are attached to the central carbon atom,it is both stabilized by hyperconjugation and +In ductive effect. The SN1 mechanism therefore dominates in reactions at tertiary alkyl centers and is further observed at secondary alkyl centers in the presence of weak nucleophiles. SN2 REACTION The SN2 reaction (also known as bimolecular nucleophilic substitution or as backside attack) is a type of nucleophilic substitution, where a lone pair from a nucleophile attacks an electron deficient electrophilic center and bonds to it, expelling another group called a leaving group. Thus the incoming group replaces the leaving group in one step. Since two reacting species are involved in the slow, rate-determining step of the reaction, this leads to the name bimolecular nucleophilic substitution, or SN2. Among inorganic chemists, the SN2 reaction is often known as the interchange mechanism. REACTION MECHANISM The reaction most often occurs at an aliphatic sp3 carbon center with an electronegative, stable leaving group attached to it X frequently a halide atom. The breaking of the C-X bond and the formation of the new C-Nu bond occur simultaneously to form a transition state in which the carbon under nucleophilic attack is pentacoordinate, and approximately sp2 hybridised. The nucleophile attacks the carbon at 180 to the leaving group, since this provides the best overlap between the nucleophiles lone pair and the C-X s* antibonding orbital. The leaving group is then pushed off the opposite side and the product is formed. If the substrate under nucleophilic attack is chiral, this can lead, although not necessarily, to an inversion of stereochemistry, called the Walden inversion. SN2 reaction of bromoethane with hydroxide ion. The products are ethanol and a bromide ion. In an example of the SN2 reaction, the attack of OH- (the nucleophile) on a bromoethane (the electrophile) results in ethanol, with bromide ejected as the leaving group. SN2 attack occurs if the backside route of attack is not sterically hindered by substituents on the substrate. Therefore this mechanism usually occurs at an unhindered primary carbon centre. If there is steric crowding on the substrate near the leaving group, such as at a tertiary carbon centre, the substitution will involve an SN1 rather than an SN2 mechanism, (an SN1 would also be more likely in this case because a sufficiently stable carbocation intermediary could be formed.) In coordination chemistry, associative substitution proceeds via a similar mechanism as SN2. FACTORS AFFECTING REACTION 1) The Basicity of the Leaving Group. By comparing the relative SN2 reaction rates of compounds with atoms in the same periodic group (the halides, for example), results show that the ability as a leaving group during an SN2 reaction depends on its basicity. In general, the weaker the basicity of a group, the greater its leaving ability. For example, the iodide ion is a very weak base and because it is so, it is the most reactive. Weak bases do not hold their electrons tightly, making it easier for their bonds to be broken. In contrast, the fluoride ion is a stronger base than the other halides and, therefore, the least reactive. In fact, the fluoride ion is such a strong base that compounds involving them essentially do not undergo SN2 reaction. Looking at the periodic table, relative basicity decreases down a group. (Stronger Base) F- > Cl- > Br- > I- (Weaker Base) 2) The Size of the Nucleophile. How readily a compound attacks an electron-deficient atom also affects an SN2 reaction. As a rule, a negatively charged species (e.g. OH -) are better nucleophiles than neutral species (e.g. H2O, water). There is a direct relationship between basicity and nucleophilicity: stronger bases are better nucleophiles. Acidity, the ability of an atom to give up a proton (H+), is comparatively relative in molecules whose attacking atoms are approximately the same in size, the weakest going toward the left side of the periodic table. If hydrogen were attached to second-row elements of the periodic table, the resulting compounds would have the following relative acidities: (Weaker Acid) NH3 If each of these acids were to give up a hydrogen, the result would be its conjugate base, and the relative strengths will reverse. The stronger base now moves toward the left side of the periodic table. (Stronger Base) -NH2 > HO- > F- (Weaker Base) Elements increase in size down the periodic table. Although basicity decreases down the periodic table, nucleophilicity increases as size increases depending on the solvent used. 3) Solvent. If a reaction is carried out in a protic solvent, whose molecules have a hydrogen bonded to an oxygen or to a nitrogen, the larger atom is a better nucleophile in an SN2 reaction. In other words, the weaker base is the better nucleophile in a protic solvent. For example, the iodide ion is better than a fluoride ion as a nucleophile. However, if the reaction is carried out in an aprotic solvent, whose molecules do not have hydrogen bonded to an oxygen or to a nitrogen, then the stronger base is the better nucleophile. In this case, the fluoride ion is better than the iodide ion as a nucleophile. 4) Sterics. Steric hindrance is any effect of a compound due to the size and/or arrangement of its substituent groups. Steric effects affect nucleophilicity but does not affect base strength. A bulky nucleophile, such as a tert-butoxide ion with its specific arrangement of methyl groups, is a poorer nucleophile than an ethoxide ion with a straighter chain of carbons, even though tert-butoxide is a stronger base.

In Jane Harrisons play Stolen Essay

In Jane Harrison’s play, ‘Stolen’, the characters of Ruby, Anne and Jimmy are utilised in order to position the audience to feel sympathetic towards those affected by the ‘Stolen Generation’. Through her plot Harrison is able to demonstrate the pain faced by the characters. Furthermore, through her script, she is also able to show the mental disintegration of the characters throughout time. Therefore, it is imperative to examine the ways in which she has used these particular facets of her play in order to rouse the emotions of the audience. Jane Harrison utilizes the script of the play ‘Stolen’ to position the audience to feel sympathy for Anne. Anne is an aboriginal female who, adopted at a young age by a white Australian family. Anne was chosen by the white couple because ‘she was by far the best’ (THE CHOSEN pg 7). Anne is seen to have a ‘good upbringing’ (THE CHOSEN pg 7) compared to the other characters in ‘Stolen’. She receives a ‘sense of security’ (THE CHOSEN pg 7) and ‘a good education’ (THE CHOSEN pg 7), but Harrison reveals to the audience that Anne has to confront problems that none of the other characters have to face. Later on in the play, Anne is confused when she asks the question ‘Am I Black or White?’ (AM I BLACK OR WHITE? pg 28). Anne is torn between her origin and the people she has been brought up with. Harrison demonstrates this theme of ‘not belonging anywhere ‘through the script. Phrases such as ‘We’ve given you everything’ (AM I BLACK OR WHITE? pg 28) opposed to ‘But we’re your real family’ (AM I BLACK OR WHITE? pg 28). Harrison creates a binary opposition between the aboriginals, Anne’s blood and race, and the white Australians. Anne is rejected from both families, thus being rejected from everyone she knows, not belonging anywhere. Therefore, Harrison presents her view to the audience that even though Anne was better of materially compared to the other characters she had to experience a different type of pain that the other characters in ‘Stolen’ do not have to encounter. Harrison portrays the message that all children of the ‘Stolen Generation’ suffered, physically and/or mentally. Harrison uses the character Ruby to show the audience how mentally affected a child from the ‘Stolen Generation’ can be. Ruby was taken away from her family at a young age, just like many other children of the ‘Stolen Generation’. Harrison positions the audience to see that Ruby had to go through hard times as a child in the orphanage. In the scene ‘UNSPOKEN ABUSE 1’ (pg 8), Ruby has come back from a weekend away with a white family, the other children are curious and ask Ruby â€Å"What else did ya do?† (UNSPOKEN ABUSE 1, pg 8), and Ruby replies with â€Å"Promised not to tell† (UNSPOKEN ABUSE 1, pg 8). The audience does not know what happened to Ruby on that weekend but by the language Harrison has used, it seems that whatever actions that occurred on that weekend had affected Ruby had changed her. Harrison shows the audience in ‘RUBY COMFORTING HER BABY’ (pg. 9), that Ruby was an ordinary girl who played ‘with her doll’ (RUBY COMFORTING HER BABY, pg. 9). This same scene also shows the audience the horrible memories that Ruby have in her young mind, as Ruby is nurturing her doll, she seems like she is pretending to be her own mother and the doll being her. Ruby tells her doll ‘I love you Ruby’ (RUBY COMFORTING HER BABY pg.9). When Ruby grows up and leaves the orphanage, she goes and works for a white family. One day, her family come to visit, but it is revealed to the audience that Ruby is mentally disabled and is not well. Ruby’s family want to take her home but Ruby replies â€Å"Don’t live in no home any more. I work for the Hardwick’s† (RUBY’S FAMILY COME TO VISIT, pg. 31). It is clearly shown that Ruby cannot see that those people are her family. Harrison displays to the audience that in Ruby’s mind, Ruby believes that she has no family, reinforcing the fact that the children of the ‘Stolen Generation’ suffered immensely. Harrison shows the audience throughout th e play, the downfall of Ruby’s mental state Jimmy is a character in the play ‘Stolen’, who is an aboriginal male who has experienced pain throughout the play and this is shown through the play with the Harrison’s use of the plot and script. The character Jimmy spent his childhood years in an orphanage. The audience see that Jimmy had been brought up with no parents, just like many of the other characters in ‘Stolen’. Like Ruby, Jimmy goes away with a white family for a weekend and comes back changed, more timid than before. Jimmy grows up and leaves the orphanage he enters a bar and some indigenous people recognised him as ‘Wajurri’ (JIMMY’S STORY, pg. 27), and they said they knew his mother. Jimmy comes to visit her but before he can meet his mother, she dies. Jimmy is so devastated that he kills himself to finally ‘go meet my mother’ (SANDY AT THE END OF THE ROAD, pg. 36). The characters Ruby, Jimmy and Anne have many similarities and differences throughout the play and Harrison uses these similarities and differences through the script and plot. Both Ruby and Anne eventually meet their families. At the end of the play, Anne is accepted by her aboriginal and white families, feeling a sense of belonging towards both of her families. The audience see that Ruby is become completely insane and even though she faces her family, she does not believe that it is her family and goes back to work. Jimmy does not get to meet his family. When Jimmy is grown up, he discovers that his mother is alive, but when he comes to visit her, he finds out that she has died. This was extremely traumatic for Jimmy, and he couldn’t endure the grief and resorts to ending his own life. Jimmy, Ruby and Anne all faced mentally enduring events at some point in their lives. But only the only happing ending out of these three characters is the one of Anne’s. Ruby’s end is a more tragic one, as she is permanently scarred from the events that have occurred to her in her life. Jimmy also suffers a tragic end as the he commits suicide. Harrison depicts the harshness and undergone by both of these characters. Ruby and Jimmy’s upbringing were very dissimilar in comparison to Anne’s, Anne living in a family that cared and provided for her, was contrasted by Harrison, towards Jimmy and Ruby’s lifestyle. Both Jimmy and Ruby were brought up in an orphanage with other aboriginal children whose families were also taken away from them. The play ‘Stolen’, written by Jane Harrison shows the audience the hardship undergone by the characters, Ruby, Jimmy and Anne. Though they are different in many aspects, these characters share the pain of not knowing where to belong and this is shown by Harrison puts forward this idea through her use of the plot and the script. WORKS CITED Harrison, Jane. Stolen. (3rd rev. Edition) Strawberry Hills: Currency Press, 2007

Who Is Hispanic An Individual Of Cuban - 928 Words

Who is Hispanic? The conceptual definition for the word Hispanic used in this paper is: an individual of Cuban, Mexican, Puerto Rican, South or Central American, or other Spanish-speaking country, culture or origin. This conception of the word Hispanic is board because it includes all people with ties to a Latin American country or country with Spanish culture, while remaining specific by maintaining that that these connections are through origin or culture. Every ten years the U.S. government issues the decennial census which collects data about the actual count of people residing in the United States. The census contains questions pertaining to race and ethnicity, and these items are collected using self-identification surveys. The ethnic and racial categories available on the census are defined by The U.S. Office of Management and Budget (OMB). When self-identifying on the census, respondents must choose the ethnic background and race(s) that most closely resemble their own. The OMB defines Latino or Hispanic as, â€Å"a person of Cuban, Mexican, Puerto Rican, South or Central American, or other Spanish culture or origin, regardless of race,† (â€Å"About-About Hispanic Origin†). The OMB mandates that government agencies use Hispanic/Latino or not Hispanic/Latino as the minimum necessary ethnicities when collecting and reporting data, (â€Å"Hispanic Origin†). Hispanic/Latino are used as a â€Å"panethnic,† (Mora) or umbrella classification in the ethnicity category and Hispanic/Latino orShow MoreRelatedThe Issue of Immigration: The Hispanic-American Diversity1456 Words   |  6 Pagesdirected toward individuals identified with Hispanic cultures (Guittierrez, 2006). There is a misconception among many Americans that all Hispanic cultures are identical in religion, linguistic, political, social, and family conventions and this misconception too often leads to biases and prejudices that create barriers. In reality, there are a many similarities between Hispanic groups but there are also many differences that characterize each group as being unique. Among the Hispanic groups thatRead MoreHispanics in the USA Essay1070 Words   |  5 PagesToday, the Hispanic population has grown tremendously over the years. We have watched the Hispanics community growth rate grow faster than any other racial and ethnic group in the nation. The Hispanic culture and community has populated all around the United States, introducing new traditions and customs. I was traveling to different to city in the States, I notice the wide spread growth of Hispanic communities, For Instance in Miami the Cuban and El Salvadoran culture is heavy populate in the areaRead MoreDiversity in the Hispanic Culture1225 Words   |  5 PagesDiversity in the Hispanic Culture Diversity in the Hispanic Culture The Hispanic community accounts for the largest minority in the United States. The United States Census Bureau reported as of July 2006, the percentage of Hispanics in the nation had grown to 15. This percentage excludes the 3.9 million Puerto Ricans whom call America their home. This number puts the United States ranking third worldwide for largest Hispanic populations, with Mexico and Colombia holding the first and secondRead MoreHispanic Culture1429 Words   |  6 PagesStates. They settled primarily in California, Florida, Massachusetts, and Washington, D.C. The civil war in Nicaragua in the year 1980 drove an estimated 800,000 Nicaraguan immigrants to the United States Mexican Americans are the most prevalent Hispanic group within the United States. Their history can be dated to four centuries within America, contrasting in different regions. In States like California, Texas, New Mexico, Arizona, and Nevada have great amounts of Mexican Americans. (MexicanRead MoreHispanic American Diversity Paper1544 Words   |  7 PagesHispanic American Diversity Paper Introduction The United States is known as the melting pot because of the many different cultures that live here. Hispanics make up 35.3 million according to the 2000 census. Many people don’t realize that within the Hispanic culture there are many different groups. The different groups have different linguistic, political, social, economic, religion, and statues. Most Hispanics see themselves in terms of their individual ethnic identity, as Mexican AmericanRead MoreThe Hispanic-American Experience1695 Words   |  7 Pagesand succeed in a land that individuals are determined to call their own. Almost every immigrant that has entered the United States has done so in hopes of finding a better life for themselves and for their families. For most Hispanic-Americans, the goal was the same. Hispanic-Americans come from a variety of different Spanish-speaking countries. Just as the wave of immigrants from Europe came to the United States of America in the late 1800s and early 1900s, Hispani cs came from places like MexicoRead MoreTest Questions: Hispanic and Latino American Diversity1305 Words   |  6 PagesWeek 7 Test- Hispanic/Latino American Diversity Part I. Each question is worth 3 points. For multiple choice questions, use highlight or bold to mark your answers. 1. The development of solidarity between ethnic subgroups, as reflected in the terms Hispanic and Asian Americans is called a. symbolic ethnicity b. ethnic solidarity c. panethnicity d. ethnic paradox 2. The common heritage of Hispanics is a. culture. b. race. c. language. d. all of these 3. At the beginningRead MoreEssay about Education Challenges Facing Hispanics in the United States1413 Words   |  6 Pages When one thinks about Hispanics, all too often the image of a field full of migrant workers picking fruit or vegetables in the hot sun comes to mind. This has become the stereotypical picture of a people whose determination and character are as strong or stronger than that of the Polish, Jewish, Greek, or Italian who arrived in the United States in the early 1900s. Then, the center of the new beginning for each immigrant family was an education. An education was the ladder by which the childrenRead MoreHispanic/Latino Culture Essay706 Words   |  3 PagesHISPANIC/LATINO CULTURE Hispanics or Latinos are defined as a people of Mexican, Puerto Rican, Cuban, South or Central American, or other Spanish speaking culture. This term â€Å"Hispanics† was created by the U.S. federal government in the early 1970’s to refer to Americans born in a Spanish speaking nation or with ancestry to Spanish territories. Hispanics people are vibrant, socializing, and fun loving people. Among various facts associated to this culture is that they have a deep sense of involvementRead MoreStereotypes Of The Latina Woman1532 Words   |  7 PagesAmerican Pro Seminar course, I noticed how unique and extraordinary each young lady in the class was. Even though they were all of Latin American descent, they differed in so many ways. I also noted that these ladies were different than the Latina individuals that I had previously seen in the media. Captivated by how intelligent and distinctive my colleagues were, I was prompted to write my research paper on the different stereo types of the Latina woman and how I perceive them to be inaccurate. Latinas

Leaders of the American Revolution

The American Revolution began in 1775 and led to the rapid formation of American armies to oppose the British. While British forces were largely led by professional officers and filled with career soldiers, the American leadership and ranks were filled with individuals drawn from all walks of colonial life. Some American leaders, such as George Washington, possessed extensive service in the militia, while others came directly from civilian life. The American leadership was also supplemented by foreign officers recruited in Europe, though these were of varying quality. During the early years of the conflict, American forces were hampered by poor generals and those who had achieved their rank through political connections. As the war wore on, many of these were replaced as competent and skilled officers emerged. American Revolution Leaders: American Major General William Alexander, Lord Stirling - noted brigade, division, and department commanderLieutenant Colonel Ethan Allen - Commander, Green Mountain Boys during 1775 attack on Fort TiconderogaMajor General Benedict Arnold - noted field commander, famously changed sides in 1780 becoming one of historys best-known traitorsCommodore John Barry - noted naval commanderBrigadier General George Rogers Clark - Conqueror of the Old NorthwestMajor General Horatio Gates - Commander, Northern Department, 1777-1778, Southern Department, 1780Colonel Christopher Greene - Commander at Fort Mercer, Battle of Red BankMajor General Nathanael Greene - Commander, Continental Army in the South (1780-1783)Commodore John Paul Jones - Key American naval commanderMajor General Henry Knox - American artillery commanderMarquis de Lafayette - Noted French volunteer in American serviceMajor General Charles Lee - Controversial American field commanderMajor General Henry Light Horse Harry Lee - noted Americ an cavalry/light infantry commanderMajor General Benjamin Lincoln - Commander, Southern Department (1778-1780)Brigadier General Francis Marion - The Swamp Fox - Noted guerilla leaderBrigadier General Hugh Mercer - American general killed in 1777Major General Richard Montgomery - Promising American general killed at the Battle of QuebecBrigadier General Daniel Morgan - Key commander at Saratoga and CowpensMajor Samuel Nicholas - Founding Officer, US Marine CorpsBrigadier General Count Casimir Pulaski - Father of the America CavalryMajor General Arthur St. Clair - Commander at Fort Ticonderoga, 1777Major General John Stark - Victor of BenningtonMajor General Baron Friedrich von Steuben - Inspector General, Continental ArmyMajor General John Sullivan - Division Commander (1776-1778), Commander, Rhode Island (1778), Sullivan Expedition (1779)General George Washington - Commander in Chief, Continental ArmyMajor General Anthony Wayne - Daring American commander who saw extensive service American Revolution Leaders - British Major John Andre - British spymasterLieutenant General John Burgoyne - British commander at the Battle of SaratogaGovernor Major General Sir Guy Carleton - British Governor of Quebec (1768-1778, commander-in-chief in America (1782-1783)General Sir Henry Clinton - British commander-in-chief in America (1778-1782)Lieutenant General Lord Charles Cornwallis - British commander in the South, forced to surrender at the Battle of YorktownMajor Patrick Ferguson - inventor of the Ferguson rifle, commander at the Battle of Kings MountainGeneral Thomas Gage - governor of Massachusetts, commander-in-chief in America (1775)Vice Admiral Richard Howe - Commander, North American Station (1776-1778)General Sir William Howe - British commander-in-chief in American (1775-1778)Admiral Lord George Rodney - British naval commanderLieutenant Colonel Banastre Tarleton - noted British cavalry commander